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

common/eth2/src/lighthouse_vc/http_client.rs

@@ -16,6 +16,7 @@ use std::path::Path;
 
 pub use reqwest;
 pub use reqwest::{Response, StatusCode, Url};
+use types::graffiti::GraffitiString;
 
 /// A wrapper around `reqwest::Client` which provides convenience methods for interfacing with a
 /// Lighthouse Validator Client HTTP server (`validator_client/src/http_api`).
@@ -467,6 +468,7 @@ impl ValidatorClientHttpClient {
         enabled: Option<bool>,
         gas_limit: Option<u64>,
         builder_proposals: Option<bool>,
+        graffiti: Option<GraffitiString>,
     ) -> Result<(), Error> {
         let mut path = self.server.full.clone();
 
@@ -482,6 +484,7 @@ impl ValidatorClientHttpClient {
                 enabled,
                 gas_limit,
                 builder_proposals,
+                graffiti,
             },
         )
         .await

common/eth2/src/lighthouse_vc/types.rs

@@ -83,6 +83,9 @@ pub struct ValidatorPatchRequest {
     #[serde(default)]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub builder_proposals: Option<bool>,
+    #[serde(default)]
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub graffiti: Option<GraffitiString>,
 }
 
 #[derive(Clone, PartialEq, Serialize, Deserialize)]