API Middleware for Keymanager Standard API Endpoints

[rauljordan]

This PR adds API middleware support for our keymanager standard https://ethereum.github.io/keymanager-APIs/#/ to support translating grpc gateway responses into the expected types, such as hex strings and enums, to align with the
standard.

Changes

• Support DELETE requests in the api middleware
• Support DELETE requests in the cors middleware used by the API gateway
• Regenerate protobufs
• Amend the custom mux used by the validator client to be able to handle the regular gateway, the api middleware, and web requests
• Small fix to delete keystores to return empty response if request has no public keys

Requirements

• GET request working! Able to return the list of public keys as hex strings being used by the keymanager
• DELETE request working! Able to successfully delete keystores and retrieve their slashing protection history. If key has been deleted before, it will still return slashing protection history and NOT_ACTIVE
• POST request working! It is able to import keystores with the given password and their associated slashing protection history as well. For a script that is able to make this complex POST request, see the Gist prepared

https://gist.github.com/rauljordan/89b35a15032dedd77cf14cfec9886c18

[james-prysm]

I think the only thing that stood out to me was that we need to remember to add put and patch as accessible functions and currently assumes put and patch as a post.. which may result in undesired effects if we don't remember to update.

[james-prysm]

didn't see anything that stood out to me and radek gave blessings

[james-prysm]

oh this looks a lot more clear