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/lib.rs

@@ -6,6 +6,7 @@ mod cli;
 mod config;
 mod duties_service;
 mod fork_service;
+mod graffiti_file;
 mod http_metrics;
 mod initialized_validators;
 mod key_cache;
@@ -304,6 +305,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             .beacon_nodes(beacon_nodes.clone())
             .runtime_context(context.service_context("block".into()))
             .graffiti(config.graffiti)
+            .graffiti_file(config.graffiti_file.clone())
             .build()?;
 
         let attestation_service = AttestationServiceBuilder::new()