Add graffiti cli flag to the validator client. (#1425)

## Issue Addressed

#1419

## Proposed Changes

Creates a `--graffiti` cli flag in the validator client. If the flag is set, it overrides graffiti in the beacon node. 

## Additional Info

beacon_node/rest_api/src/validator.rs

@@ -1,4 +1,6 @@
-use crate::helpers::{check_content_type_for_json, publish_beacon_block_to_network};
+use crate::helpers::{
+    check_content_type_for_json, parse_hex_ssz_bytes, publish_beacon_block_to_network,
+};
 use crate::response_builder::ResponseBuilder;
 use crate::{ApiError, ApiResult, NetworkChannel, UrlQuery};
 use beacon_chain::{
@@ -288,8 +290,14 @@ pub fn get_new_beacon_block<T: BeaconChainTypes>(
     let slot = query.slot()?;
     let randao_reveal = query.randao_reveal()?;
 
+    let validator_graffiti = if let Some((_key, value)) = query.first_of_opt(&["graffiti"]) {
+        Some(parse_hex_ssz_bytes(&value)?)
+    } else {
+        None
+    };
+
     let (new_block, _state) = beacon_chain
-        .produce_block(randao_reveal, slot)
+        .produce_block(randao_reveal, slot, validator_graffiti)
         .map_err(|e| {
             error!(
                 log,

beacon_node/rest_api/tests/test.rs

@@ -460,7 +460,7 @@ fn validator_block_post() {
             remote_node
                 .http
                 .validator()
-                .produce_block(slot, randao_reveal),
+                .produce_block(slot, randao_reveal, None),
         )
         .expect("should fetch block from http api");
 
@@ -547,7 +547,7 @@ fn validator_block_get() {
             remote_node
                 .http
                 .validator()
-                .produce_block(slot, randao_reveal.clone()),
+                .produce_block(slot, randao_reveal.clone(), None),
         )
         .expect("should fetch block from http api");
 
@@ -555,7 +555,50 @@ fn validator_block_get() {
         .client
         .beacon_chain()
         .expect("client should have beacon chain")
-        .produce_block(randao_reveal, slot)
+        .produce_block(randao_reveal, slot, None)
+        .expect("should produce block");
+
+    assert_eq!(
+        block, expected_block,
+        "the block returned from the API should be as expected"
+    );
+}
+
+#[test]
+fn validator_block_get_with_graffiti() {
+    let mut env = build_env();
+
+    let spec = &E::default_spec();
+
+    let node = build_node(&mut env, testing_client_config());
+    let remote_node = node.remote_node().expect("should produce remote node");
+
+    let beacon_chain = node
+        .client
+        .beacon_chain()
+        .expect("client should have beacon chain");
+
+    let slot = Slot::new(1);
+    let randao_reveal = get_randao_reveal(beacon_chain, slot, spec);
+
+    let block = env
+        .runtime()
+        .block_on(remote_node.http.validator().produce_block(
+            slot,
+            randao_reveal.clone(),
+            Some(*b"test-graffiti-test-graffiti-test"),
+        ))
+        .expect("should fetch block from http api");
+
+    let (expected_block, _state) = node
+        .client
+        .beacon_chain()
+        .expect("client should have beacon chain")
+        .produce_block(
+            randao_reveal,
+            slot,
+            Some(*b"test-graffiti-test-graffiti-test"),
+        )
         .expect("should produce block");
 
     assert_eq!(