verifying remote signing / web3signer

### Problem description

Lodestar already implements the w3signer standard https://chainsafe.github.io/lodestar/run/validator-management/external-signer

The standard assumes the w3s trusts the validator client, since there is no way for the w3s to verify the payload to be signed.

### Solution description

The proposal to fix that is: https://github.com/ethereum/remote-signing-api/pull/10

Nimbus has implemented this: https://nimbus.guide/web3signer.html#verifying-web3signer, it is specially useful for diva which distributes the w3s signatures.