verifying remote signing / web3signer

[franjoespejo]

Teku already implements the w3signer standard https://docs.teku.consensys.io/how-to/use-external-signer/use-web3signer.

The standard assumes the w3s trusts the validator client, since there is no way for the w3s to verify the payload to be signed.
The proposal to fix that is: ethereum/remote-signing-api#10,

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