Key manager API: implement standard Graffiti management APIs

[jimmygchen]

Description

New GET, POST, DELETE graffiti endpoints are being added to Key Manager API spec and needs to be implemented:
ethereum/keymanager-APIs#63

Lighthouse already implement some custom APIs to manage graffitis:

• GET /lighthouse/ui/graffiti
• PATCH /lighthouse/validators/:voting_pubkey

These existing endpoints should remain unchanged as they are used by the Siren UI and are slightly different to the standard APIs. For the new POST endpoint, we should implement a similar logic when updating the validator graffiti to the existing PATCH endpoint:

This endpoint updates the graffiti in both the validator definition file and the in memory InitializedValidators. In the next block proposal, the new graffiti will be used.

Note that the --graffiti-file 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).

^ and perhaps return 403 instead of 400 to be more spec compliant.

[eserilev]

I'd like to pick this up

[jimmygchen]

That's great, thanks @eserilev !

[jimmygchen]

Completed in #4951 , thanks @eserilev !