Feature: auto-reload validators from Web3Signer

[michaelsproul]

Description

@guybrush points out that web3signer has an endpoint like /api/v1/eth2/publicKey for fetching the list of available validators.

The Lighthouse VC could (optionally) poll this endpoint to auto-enable validators.

[paulhauner]

I did notice that endpoint during implementation and decided to leave it alone. My reasoning was that if someone wants to add validators then they should call that endpoint themselves. If they can add validators to web3signer then I assume they're also privileged enough to call its HTTP API.

At the time it seemed like a bit of an over-reach for LH to start having an opinion on when new validators should be added or not. I still hold that opinion now, but I could certainly change my mind if there's some sort of flow/scenario that I haven't considered.