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.

lighthouse/tests/account_manager.rs

@@ -482,6 +482,7 @@ fn validator_import_launchpad() {
     let expected_def = ValidatorDefinition {
         enabled: true,
         description: "".into(),
+        graffiti: None,
         voting_public_key: keystore.public_key().unwrap(),
         signing_definition: SigningDefinition::LocalKeystore {
             voting_keystore_path,