Feature: Implement keymanager graffiti API endpoint

[yorickdowne]

Is your feature request related to a problem? Please describe.

Users cannot set a per-validator graffiti without manually changing json files

Describe the solution you'd like

Implement keymanager graffiti endpoints so user can use ethd keys get|set|delete-graffiti

Additional context

Very similar to the gas-limit endpoint. See https://ethereum.github.io/keymanager-APIs/?urls.primaryName=dev#/Graffiti

[xrchz]

For reference what are the JSON files we can edit as a workaround in the meantime? (If the validator is imported via API)

[tersec]

For reference what are the JSON files we can edit as a workaround in the meantime? (If the validator is imported via API)

Either keystore.json or remote_keystore.json, depending on whether it's a local or remote keystore.

[tersec]

#6054