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/lib.rs

@@ -30,13 +30,14 @@ use crate::beacon_node_fallback::{
     RequireSynced,
 };
 use crate::doppelganger_service::DoppelgangerService;
+use crate::graffiti_file::GraffitiFile;
 use account_utils::validator_definitions::ValidatorDefinitions;
 use attestation_service::{AttestationService, AttestationServiceBuilder};
 use block_service::{BlockService, BlockServiceBuilder};
 use clap::ArgMatches;
 use duties_service::DutiesService;
 use environment::RuntimeContext;
-use eth2::{reqwest::ClientBuilder, BeaconNodeHttpClient, StatusCode, Timeouts};
+use eth2::{reqwest::ClientBuilder, types::Graffiti, BeaconNodeHttpClient, StatusCode, Timeouts};
 use http_api::ApiSecret;
 use notifier::spawn_notifier;
 use parking_lot::RwLock;
@@ -57,7 +58,7 @@ use tokio::{
     sync::mpsc,
     time::{sleep, Duration},
 };
-use types::{EthSpec, Hash256};
+use types::{EthSpec, Hash256, PublicKeyBytes};
 use validator_store::ValidatorStore;
 
 /// The interval between attempts to contact the beacon node during startup.
@@ -526,6 +527,8 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
                 api_secret,
                 validator_store: Some(self.validator_store.clone()),
                 validator_dir: Some(self.config.validator_dir.clone()),
+                graffiti_file: self.config.graffiti_file.clone(),
+                graffiti_flag: self.config.graffiti,
                 spec: self.context.eth2_config.spec.clone(),
                 config: self.config.http_api.clone(),
                 log: log.clone(),
@@ -726,3 +729,24 @@ pub fn load_pem_certificate<P: AsRef<Path>>(pem_path: P) -> Result<Certificate,
         .map_err(|e| format!("Unable to read certificate file: {}", e))?;
     Certificate::from_pem(&buf).map_err(|e| format!("Unable to parse certificate: {}", e))
 }
+
+// Given the various graffiti control methods, determine the graffiti that will be used for
+// the next block produced by the validator with the given public key.
+pub fn determine_graffiti(
+    validator_pubkey: &PublicKeyBytes,
+    log: &Logger,
+    graffiti_file: Option<GraffitiFile>,
+    validator_definition_graffiti: Option<Graffiti>,
+    graffiti_flag: Option<Graffiti>,
+) -> Option<Graffiti> {
+    graffiti_file
+        .and_then(|mut g| match g.load_graffiti(validator_pubkey) {
+            Ok(g) => g,
+            Err(e) => {
+                warn!(log, "Failed to read graffiti file"; "error" => ?e);
+                None
+            }
+        })
+        .or(validator_definition_graffiti)
+        .or(graffiti_flag)
+}