Add API endpoint to get VC graffiti (#3779)

## Issue Addressed

#3766

## Proposed Changes

Adds an endpoint to get the graffiti that will be used for the next block proposal for each validator.

## Usage
```bash
curl -H "Authorization: Bearer api-token" http://localhost:9095/lighthouse/ui/graffiti | jq
```

```json
{
  "data": {
    "0x81283b7a20e1ca460ebd9bbd77005d557370cabb1f9a44f530c4c4c66230f675f8df8b4c2818851aa7d77a80ca5a4a5e": "mr f was here",
    "0xa3a32b0f8b4ddb83f1a0a853d81dd725dfe577d4f4c3db8ece52ce2b026eca84815c1a7e8e92a4de3d755733bf7e4a9b": "mr v was here",
    "0x872c61b4a7f8510ec809e5b023f5fdda2105d024c470ddbbeca4bc74e8280af0d178d749853e8f6a841083ac1b4db98f": null
  }
}
```

## Additional Info

This will only return graffiti that the validator client knows about.
That is from these 3 sources:
1. Graffiti File
2. validator_definitions.yml
3. The `--graffiti` flag on the VC

If the graffiti is set on the BN, it will not be returned. This may warrant an additional endpoint on the BN side which can be used in the event the endpoint returns `null`.

validator_client/src/http_api/mod.rs

@@ -4,7 +4,7 @@ mod keystores;
 mod remotekeys;
 mod tests;
 
-use crate::ValidatorStore;
+use crate::{determine_graffiti, GraffitiFile, ValidatorStore};
 use account_utils::{
     mnemonic_from_phrase,
     validator_definitions::{SigningDefinition, ValidatorDefinition, Web3SignerDefinition},
@@ -13,13 +13,14 @@ pub use api_secret::ApiSecret;
 use create_validator::{create_validators_mnemonic, create_validators_web3signer};
 use eth2::lighthouse_vc::{
     std_types::{AuthResponse, GetFeeRecipientResponse, GetGasLimitResponse},
-    types::{self as api_types, GenericResponse, PublicKey, PublicKeyBytes},
+    types::{self as api_types, GenericResponse, Graffiti, PublicKey, PublicKeyBytes},
 };
 use lighthouse_version::version_with_platform;
 use parking_lot::RwLock;
 use serde::{Deserialize, Serialize};
 use slog::{crit, info, warn, Logger};
 use slot_clock::SlotClock;
+use std::collections::HashMap;
 use std::future::Future;
 use std::marker::PhantomData;
 use std::net::{IpAddr, Ipv4Addr, SocketAddr};
@@ -65,6 +66,8 @@ pub struct Context<T: SlotClock, E: EthSpec> {
     pub api_secret: ApiSecret,
     pub validator_store: Option<Arc<ValidatorStore<T, E>>>,
     pub validator_dir: Option<PathBuf>,
+    pub graffiti_file: Option<GraffitiFile>,
+    pub graffiti_flag: Option<Graffiti>,
     pub spec: ChainSpec,
     pub config: Config,
     pub log: Logger,
@@ -177,6 +180,12 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
             })
         });
 
+    let inner_graffiti_file = ctx.graffiti_file.clone();
+    let graffiti_file_filter = warp::any().map(move || inner_graffiti_file.clone());
+
+    let inner_graffiti_flag = ctx.graffiti_flag;
+    let graffiti_flag_filter = warp::any().map(move || inner_graffiti_flag);
+
     let inner_ctx = ctx.clone();
     let log_filter = warp::any().map(move || inner_ctx.log.clone());
 
@@ -329,6 +338,42 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
             })
         });
 
+    let get_lighthouse_ui_graffiti = warp::path("lighthouse")
+        .and(warp::path("ui"))
+        .and(warp::path("graffiti"))
+        .and(warp::path::end())
+        .and(validator_store_filter.clone())
+        .and(graffiti_file_filter)
+        .and(graffiti_flag_filter)
+        .and(signer.clone())
+        .and(log_filter.clone())
+        .and_then(
+            |validator_store: Arc<ValidatorStore<T, E>>,
+             graffiti_file: Option<GraffitiFile>,
+             graffiti_flag: Option<Graffiti>,
+             signer,
+             log| {
+                blocking_signed_json_task(signer, move || {
+                    let mut result = HashMap::new();
+                    for (key, graffiti_definition) in validator_store
+                        .initialized_validators()
+                        .read()
+                        .get_all_validators_graffiti()
+                    {
+                        let graffiti = determine_graffiti(
+                            key,
+                            &log,
+                            graffiti_file.clone(),
+                            graffiti_definition,
+                            graffiti_flag,
+                        );
+                        result.insert(key.to_string(), graffiti.map(|g| g.as_utf8_lossy()));
+                    }
+                    Ok(api_types::GenericResponse::from(result))
+                })
+            },
+        );
+
     // POST lighthouse/validators/
     let post_validators = warp::path("lighthouse")
         .and(warp::path("validators"))
@@ -945,6 +990,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                         .or(get_lighthouse_validators)
                         .or(get_lighthouse_validators_pubkey)
                         .or(get_lighthouse_ui_health)
+                        .or(get_lighthouse_ui_graffiti)
                         .or(get_fee_recipient)
                         .or(get_gas_limit)
                         .or(get_std_keystores)

validator_client/src/http_api/tests.rs

@@ -120,6 +120,8 @@ impl ApiTester {
             api_secret,
             validator_dir: Some(validator_dir.path().into()),
             validator_store: Some(validator_store.clone()),
+            graffiti_file: None,
+            graffiti_flag: Some(Graffiti::default()),
             spec: E::default_spec(),
             config: HttpConfig {
                 enabled: true,