Add support for updating validator graffiti (#4417)

## Issue Addressed

#4386 

## Proposed Changes

The original proposal described in the issue adds a new endpoint to support updating validator graffiti, but I realized we already have an endpoint that we use for updating various validator fields in memory and in the validator definitions file, so I think that would be the best place to add this to.

### API endpoint

`PATCH lighthouse/validators/{validator_pubkey}` 

This endpoint updates the graffiti in both the [ validator definition file](https://lighthouse-book.sigmaprime.io/graffiti.html#2-setting-the-graffiti-in-the-validator_definitionsyml) and the in memory `InitializedValidators`. In the next block proposal, the new graffiti will be used.

Note that the [`--graffiti-file`](https://lighthouse-book.sigmaprime.io/graffiti.html#1-using-the---graffiti-file-flag-on-the-validator-client) flag has a priority over the validator definitions file, so if the caller attempts to update the graffiti while the `--graffiti-file` flag is present, the endpoint will return an error (Bad request 400).

## Tasks

- [x] Add graffiti update support to `PATCH lighthouse/validators/{validator_pubkey}` 
- [x] Return error if user tries to update graffiti while the `--graffiti-flag` is set
- [x] Update Lighthouse book

validator_client/src/initialized_validators.rs

@@ -27,6 +27,7 @@ use std::io::{self, Read};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 use std::time::Duration;
+use types::graffiti::GraffitiString;
 use types::{Address, Graffiti, Keypair, PublicKey, PublicKeyBytes};
 use url::{ParseError, Url};
 use validator_dir::Builder as ValidatorDirBuilder;
@@ -147,6 +148,10 @@ impl InitializedValidator {
     pub fn get_index(&self) -> Option<u64> {
         self.index
     }
+
+    pub fn get_graffiti(&self) -> Option<Graffiti> {
+        self.graffiti
+    }
 }
 
 fn open_keystore(path: &Path) -> Result<Keystore, Error> {
@@ -671,8 +676,8 @@ impl InitializedValidators {
         self.validators.get(public_key)
     }
 
-    /// Sets the `InitializedValidator` and `ValidatorDefinition` `enabled`, `gas_limit`, and `builder_proposals`
-    /// values.
+    /// Sets the `InitializedValidator` and `ValidatorDefinition` `enabled`, `gas_limit`,
+    /// `builder_proposals`, and `graffiti` values.
     ///
     /// ## Notes
     ///
@@ -682,7 +687,7 @@ impl InitializedValidators {
     ///
     /// If a `gas_limit` is included in the call to this function, it will also be updated and saved
     /// to disk. If `gas_limit` is `None` the `gas_limit` *will not* be unset in `ValidatorDefinition`
-    /// or `InitializedValidator`. The same logic applies to `builder_proposals`.
+    /// or `InitializedValidator`. The same logic applies to `builder_proposals` and `graffiti`.
     ///
     /// Saves the `ValidatorDefinitions` to file, even if no definitions were changed.
     pub async fn set_validator_definition_fields(
@@ -691,6 +696,7 @@ impl InitializedValidators {
         enabled: Option<bool>,
         gas_limit: Option<u64>,
         builder_proposals: Option<bool>,
+        graffiti: Option<GraffitiString>,
     ) -> Result<(), Error> {
         if let Some(def) = self
             .definitions
@@ -708,6 +714,9 @@ impl InitializedValidators {
             if let Some(builder_proposals) = builder_proposals {
                 def.builder_proposals = Some(builder_proposals);
             }
+            if let Some(graffiti) = graffiti.clone() {
+                def.graffiti = Some(graffiti);
+            }
         }
 
         self.update_validators().await?;
@@ -723,6 +732,9 @@ impl InitializedValidators {
             if let Some(builder_proposals) = builder_proposals {
                 val.builder_proposals = Some(builder_proposals);
             }
+            if let Some(graffiti) = graffiti {
+                val.graffiti = Some(graffiti.into());
+            }
         }
 
         self.definitions