Implement VC API (#1657)

## Issue Addressed

NA

## Proposed Changes

- Implements a HTTP API for the validator client.
- Creates EIP-2335 keystores with an empty `description` field, instead of a missing `description` field. Adds option to set name.
- Be more graceful with setups without any validators (yet)
    - Remove an error log when there are no validators.
    - Create the `validator` dir if it doesn't exist.
- Allow building a `ValidatorDir` without a withdrawal keystore (required for the API method where we only post a voting keystore).
- Add optional `description` field to `validator_definitions.yml`

## TODO

- [x] Signature header, as per https://github.com/sigp/lighthouse/issues/1269#issuecomment-649879855
- [x] Return validator descriptions
- [x] Return deposit data
- [x] Respect the mnemonic offset
- [x] Check that mnemonic can derive returned keys
- [x] Be strict about non-localhost
- [x] Allow graceful start without any validators (+ create validator dir)
- [x] Docs final pass
- [x] Swap to EIP-2335 description field. 
- [x] Fix Zerioze TODO in VC api types.
- [x] Zeroize secp256k1 key

## Endpoints

- [x] `GET /lighthouse/version`
- [x] `GET /lighthouse/health`
- [x] `GET /lighthouse/validators` 
- [x] `POST /lighthouse/validators/hd`
- [x] `POST /lighthouse/validators/keystore`
- [x] `PATCH /lighthouse/validators/:validator_pubkey`
- [ ] ~~`POST /lighthouse/validators/:validator_pubkey/exit/:epoch`~~ Future works


## Additional Info

TBC

validator_client/src/lib.rs

@@ -10,6 +10,8 @@ mod notifier;
 mod validator_duty;
 mod validator_store;
 
+pub mod http_api;
+
 pub use cli::cli_app;
 pub use config::Config;
 
@@ -22,11 +24,14 @@ use environment::RuntimeContext;
 use eth2::{reqwest::ClientBuilder, BeaconNodeHttpClient, StatusCode, Url};
 use fork_service::{ForkService, ForkServiceBuilder};
 use futures::channel::mpsc;
+use http_api::ApiSecret;
 use initialized_validators::InitializedValidators;
 use notifier::spawn_notifier;
 use slog::{error, info, Logger};
 use slot_clock::SlotClock;
 use slot_clock::SystemTimeSlotClock;
+use std::marker::PhantomData;
+use std::net::SocketAddr;
 use std::sync::Arc;
 use std::time::{SystemTime, UNIX_EPOCH};
 use tokio::time::{delay_for, Duration};
@@ -42,9 +47,11 @@ const HTTP_TIMEOUT: Duration = Duration::from_secs(12);
 pub struct ProductionValidatorClient<T: EthSpec> {
     context: RuntimeContext<T>,
     duties_service: DutiesService<SystemTimeSlotClock, T>,
-    fork_service: ForkService<SystemTimeSlotClock, T>,
+    fork_service: ForkService<SystemTimeSlotClock>,
     block_service: BlockService<SystemTimeSlotClock, T>,
     attestation_service: AttestationService<SystemTimeSlotClock, T>,
+    validator_store: ValidatorStore<SystemTimeSlotClock, T>,
+    http_api_listen_addr: Option<SocketAddr>,
     config: Config,
 }
 
@@ -55,7 +62,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         context: RuntimeContext<T>,
         cli_args: &ArgMatches<'_>,
     ) -> Result<Self, String> {
-        let config = Config::from_cli(&cli_args)
+        let config = Config::from_cli(&cli_args, context.log())
             .map_err(|e| format!("Unable to initialize config: {}", e))?;
         Self::new(context, config).await
     }
@@ -68,7 +75,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         info!(
             log,
             "Starting validator client";
-            "beacon_node" => &config.http_server,
+            "beacon_node" => &config.beacon_node,
             "validator_dir" => format!("{:?}", config.validator_dir),
         );
 
@@ -106,7 +113,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         );
 
         let beacon_node_url: Url = config
-            .http_server
+            .beacon_node
             .parse()
             .map_err(|e| format!("Unable to parse beacon node URL: {:?}", e))?;
         let beacon_node_http_client = ClientBuilder::new()
@@ -144,7 +151,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         let fork_service = ForkServiceBuilder::new()
             .slot_clock(slot_clock.clone())
             .beacon_node(beacon_node.clone())
-            .runtime_context(context.service_context("fork".into()))
+            .log(log.clone())
             .build()?;
 
         let validator_store: ValidatorStore<SystemTimeSlotClock, T> = ValidatorStore::new(
@@ -183,7 +190,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         let attestation_service = AttestationServiceBuilder::new()
             .duties_service(duties_service.clone())
             .slot_clock(slot_clock)
-            .validator_store(validator_store)
+            .validator_store(validator_store.clone())
             .beacon_node(beacon_node)
             .runtime_context(context.service_context("attestation".into()))
             .build()?;
@@ -194,7 +201,9 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             fork_service,
             block_service,
             attestation_service,
+            validator_store,
             config,
+            http_api_listen_addr: None,
         })
     }
 
@@ -204,6 +213,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         // whole epoch!
         let channel_capacity = T::slots_per_epoch() as usize;
         let (block_service_tx, block_service_rx) = mpsc::channel(channel_capacity);
+        let log = self.context.log();
 
         self.duties_service
             .clone()
@@ -215,7 +225,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
 
         self.fork_service
             .clone()
-            .start_update_service(&self.context.eth2_config.spec)
+            .start_update_service(&self.context)
             .map_err(|e| format!("Unable to start fork service: {}", e))?;
 
         self.block_service
@@ -230,6 +240,35 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
 
         spawn_notifier(self).map_err(|e| format!("Failed to start notifier: {}", e))?;
 
+        let api_secret = ApiSecret::create_or_open(&self.config.validator_dir)?;
+
+        self.http_api_listen_addr = if self.config.http_api.enabled {
+            let ctx: Arc<http_api::Context<SystemTimeSlotClock, T>> = Arc::new(http_api::Context {
+                api_secret,
+                validator_store: Some(self.validator_store.clone()),
+                validator_dir: Some(self.config.validator_dir.clone()),
+                spec: self.context.eth2_config.spec.clone(),
+                config: self.config.http_api.clone(),
+                log: log.clone(),
+                _phantom: PhantomData,
+            });
+
+            let exit = self.context.executor.exit();
+
+            let (listen_addr, server) = http_api::serve(ctx, exit)
+                .map_err(|e| format!("Unable to start HTTP API server: {:?}", e))?;
+
+            self.context
+                .clone()
+                .executor
+                .spawn_without_exit(async move { server.await }, "http-api");
+
+            Some(listen_addr)
+        } else {
+            info!(log, "HTTP API server is disabled");
+            None
+        };
+
         Ok(())
     }
 }