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

validator_client/src/block_service.rs

@@ -7,7 +7,7 @@ use slog::{crit, debug, error, info, trace, warn};
 use slot_clock::SlotClock;
 use std::ops::Deref;
 use std::sync::Arc;
-use types::{EthSpec, PublicKey, Slot};
+use types::{EthSpec, Graffiti, PublicKey, Slot};
 
 /// Builds a `BlockService`.
 pub struct BlockServiceBuilder<T, E: EthSpec> {
@@ -15,6 +15,7 @@ pub struct BlockServiceBuilder<T, E: EthSpec> {
     slot_clock: Option<Arc<T>>,
     beacon_node: Option<RemoteBeaconNode<E>>,
     context: Option<RuntimeContext<E>>,
+    graffiti: Option<Graffiti>,
 }
 
 impl<T: SlotClock + 'static, E: EthSpec> BlockServiceBuilder<T, E> {
@@ -24,6 +25,7 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockServiceBuilder<T, E> {
             slot_clock: None,
             beacon_node: None,
             context: None,
+            graffiti: None,
         }
     }
 
@@ -47,6 +49,11 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockServiceBuilder<T, E> {
         self
     }
 
+    pub fn graffiti(mut self, graffiti: Option<Graffiti>) -> Self {
+        self.graffiti = graffiti;
+        self
+    }
+
     pub fn build(self) -> Result<BlockService<T, E>, String> {
         Ok(BlockService {
             inner: Arc::new(Inner {
@@ -62,6 +69,7 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockServiceBuilder<T, E> {
                 context: self
                     .context
                     .ok_or_else(|| "Cannot build BlockService without runtime_context")?,
+                graffiti: self.graffiti,
             }),
         })
     }
@@ -73,6 +81,7 @@ pub struct Inner<T, E: EthSpec> {
     slot_clock: Arc<T>,
     beacon_node: RemoteBeaconNode<E>,
     context: RuntimeContext<E>,
+    graffiti: Option<Graffiti>,
 }
 
 /// Attempts to produce attestations for any block producer(s) at the start of the epoch.
@@ -214,7 +223,7 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockService<T, E> {
             .beacon_node
             .http
             .validator()
-            .produce_block(slot, randao_reveal)
+            .produce_block(slot, randao_reveal, self.graffiti)
             .await
             .map_err(|e| format!("Error from beacon node when producing block: {:?}", e))?;