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.

book/src/SUMMARY.md

@@ -30,6 +30,7 @@
 	* [Prometheus Metrics](./advanced_metrics.md)
 * [Advanced Usage](./advanced.md)
     * [Custom Data Directories](./advanced-datadir.md)
+    * [Validator Graffiti](./graffiti.md)
     * [Database Configuration](./advanced_database.md)
 	* [Local Testnets](./local-testnets.md)
     * [Advanced Networking](./advanced_networking.md)

book/src/api-vc-endpoints.md

@@ -279,7 +279,8 @@ Typical Responses | 200
     {
         "enable": true,
         "description": "validator_one",
-        "deposit_gwei": "32000000000"
+        "deposit_gwei": "32000000000",
+        "graffiti": "Mr F was here"
     },
     {
         "enable": false,

book/src/graffiti.md

@@ -0,0 +1,62 @@
+# Validator Graffiti
+
+Lighthouse provides four options for setting validator graffiti.
+
+### 1. Using the "--graffiti-file" flag on the validator client
+Users can specify a file with the `--graffiti-file` flag. This option is useful for dynamically changing graffitis for various use cases (e.g. drawing on the beaconcha.in graffiti wall). This file is loaded once on startup and reloaded everytime a validator is chosen to propose a block.
+
+Usage:
+`lighthouse vc --graffiti-file graffiti_file.txt`
+
+The file should contain key value pairs corresponding to validator public keys and their associated graffiti. The file can also contain a `default` key for the default case.
+```
+default: default_graffiti
+public_key1: graffiti1
+public_key2: graffiti2
+...
+```
+
+Below is an example of a graffiti file:
+
+```
+default: Lighthouse
+0x87a580d31d7bc69069b55f5a01995a610dd391a26dc9e36e81057a17211983a79266800ab8531f21f1083d7d84085007: mr f was here
+0xa5566f9ec3c6e1fdf362634ebec9ef7aceb0e460e5079714808388e5d48f4ae1e12897fed1bea951c17fa389d511e477: mr v was here
+```
+
+Lighthouse will first search for the graffiti corresponding to the public key of the proposing validator, if there are no matches for the public key, then it uses the graffiti corresponding to the default key if present.
+
+### 2. Setting the graffiti in the `validator_definitions.yml`
+Users can set validator specific graffitis in `validator_definitions.yml` with the `graffiti` key. This option is recommended for static setups where the graffitis won't change on every new block proposal.
+
+Below is an example of the validator_definitions.yml with validator specific graffitis:
+```
+---
+- enabled: true
+  voting_public_key: "0x87a580d31d7bc69069b55f5a01995a610dd391a26dc9e36e81057a17211983a79266800ab8531f21f1083d7d84085007"
+  type: local_keystore
+  voting_keystore_path: /home/paul/.lighthouse/validators/0x87a580d31d7bc69069b55f5a01995a610dd391a26dc9e36e81057a17211983a79266800ab8531f21f1083d7d84085007/voting-keystore.json
+  voting_keystore_password_path: /home/paul/.lighthouse/secrets/0x87a580d31d7bc69069b55f5a01995a610dd391a26dc9e36e81057a17211983a79266800ab8531f21f1083d7d84085007
+  graffiti: "mr f was here"
+- enabled: false
+  voting_public_key: "0xa5566f9ec3c6e1fdf362634ebec9ef7aceb0e460e5079714808388e5d48f4ae1e12897fed1bea951c17fa389d511e477"
+  type: local_keystore
+  voting_keystore_path: /home/paul/.lighthouse/validators/0xa5566f9ec3c6e1fdf362634ebec9ef7aceb0e460e5079714808388e5d48f4ae1e12897fed1bea951c17fa389d511e477/voting-keystore.json
+  voting_keystore_password: myStrongpa55word123&$
+  graffiti: "somethingprofound"
+```
+
+### 3. Using the "--graffiti" flag on the validator client
+Users can specify a common graffiti for all their validators using the `--graffiti` flag on the validator client.
+
+### 4. Using the "--graffiti" flag on the beacon node
+Users can also specify a common graffiti using the `--graffiti` flag on the beacon node as a common  graffiti for all validators.
+
+Usage: `lighthouse vc --graffiti fortytwo`
+
+> Note: The order of preference for loading the graffiti is as follows:
+> 1. Read from `--graffiti-file` if provided.
+> 2. If `--graffiti-file` is not provided or errors, read graffiti from `validator_definitions.yml`.
+> 3. If graffiti is not specified in `validator_definitions.yml`, load the graffiti passed in the `--graffiti` flag on the validator client.
+> 4. If the `--graffiti` flag on the validator client is not passed, load the graffiti passed in the `--graffiti` flag on the beacon node.
+> 4. If the `--graffiti` flag is not passed, load the default Lighthouse graffiti.