Add (lack of) REST implementation for GetFeeRecipientByPubKey

[nalepae]

What type of PR is this?

Feature

Related gitHub issue:

#11580

How other validator clients implementation deals with this issue: ethereum/keymanager-APIs#55

What does this PR do? Why is it needed?

This PR deals with the (lack of) implementation for the GetFeeRecipientByPubKey method in beacon API.

With gRPC implementation, if, for a given {pubkey}:

• no {pubkey} specific fee recipient is set in validator client, AND
• no default fee recipient is set in validator client, AND
• the GetFeeRecipientByPubKey gRPC beacon API call responds on error, then:

Before this PR, the call to GET /eth/v1/validator/{pubkey}/feerecipient returns the burn address 0x0000....
After this PR the call to GET /eth/v1/validator/{pubkey}/feerecipient returns:

{
    "message": "Failed to retrieve default fee recipient from beacon node",
    "code": 500
}

With gRPC implementation, if, for a given {pubkey}:

• no {pubkey} specific fee recipient is set in validator client, AND
• no default fee recipient is set in validator client, AND
• the GetFeeRecipientByPubKey gRPC beacon API call responds with a nil value or with an empty value, then:

Before this PR, the call to GET /eth/v1/validator/{pubkey}/feerecipient returns the burn address 0x0000....
After this PR the call to GET /eth/v1/validator/{pubkey}/feerecipient returns:

{
    "message": "No fee recipient set",
    "code": 400
}

With beacon API implementation, if, for a given {pubkey}:

• no {pubkey} specific fee recipient is set in validator client, AND
• no default fee recipient is set in validator client, then:

the call to GET /eth/v1/validator/{pubkey}/feerecipient returns:

{
    "message": "No fee recipient set",
    "code": 400
}

Warning:
The 400 and 500 code listed above will cause, in Prysm validator WebUI, the following error:

Edit:
After prysmaticlabs/prysm-web-ui#242:

[james-prysm]

Depends on web UI changes, will track those first and tie back to this PR

[nalepae]

After prysmaticlabs/prysm-web-ui#242: