Set graffiti per validator (#2044)

## Issue Addressed

Resolves #1944 

## Proposed Changes

Adds a "graffiti" key to the `validator_definitions.yml`. Setting the key will override anything passed through the validator `--graffiti` flag. 
Returns an error if the value for the graffiti key is > 32 bytes instead of silently truncating.

validator_client/src/http_api/create_validator.rs

@@ -133,7 +133,12 @@ pub async fn create_validators<P: AsRef<Path>, T: 'static + SlotClock, E: EthSpe
         drop(validator_dir);
 
         validator_store
-            .add_validator_keystore(voting_keystore_path, voting_password_string, request.enable)
+            .add_validator_keystore(
+                voting_keystore_path,
+                voting_password_string,
+                request.enable,
+                request.graffiti.clone(),
+            )
             .await
             .map_err(|e| {
                 warp_utils::reject::custom_server_error(format!(
@@ -145,6 +150,7 @@ pub async fn create_validators<P: AsRef<Path>, T: 'static + SlotClock, E: EthSpe
         validators.push(api_types::CreatedValidator {
             enabled: request.enable,
             description: request.description.clone(),
+            graffiti: request.graffiti.clone(),
             voting_pubkey,
             eth1_deposit_tx_data: serde_utils::hex::encode(&eth1_deposit_data.rlp),
             deposit_gwei: request.deposit_gwei,

validator_client/src/http_api/mod.rs

@@ -383,6 +383,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                     let voting_keystore_path = validator_dir.voting_keystore_path();
                     drop(validator_dir);
                     let voting_password = body.password.clone();
+                    let graffiti = body.graffiti.clone();
 
                     let validator_def = {
                         if let Some(runtime) = runtime.upgrade() {
@@ -391,6 +392,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                                     voting_keystore_path,
                                     voting_password,
                                     body.enable,
+                                    graffiti,
                                 ))
                                 .map_err(|e| {
                                     warp_utils::reject::custom_server_error(format!(

validator_client/src/http_api/tests.rs

@@ -210,6 +210,7 @@ impl ApiTester {
             .map(|i| ValidatorRequest {
                 enable: !s.disabled.contains(&i),
                 description: format!("boi #{}", i),
+                graffiti: None,
                 deposit_gwei: E::default_spec().max_effective_balance,
             })
             .collect::<Vec<_>>();
@@ -339,6 +340,7 @@ impl ApiTester {
                     .unwrap()
                     .into(),
                 keystore,
+                graffiti: None,
             };
 
             self.client
@@ -355,6 +357,7 @@ impl ApiTester {
                 .unwrap()
                 .into(),
             keystore,
+            graffiti: None,
         };
 
         let response = self