Adds the `use_remote_signer` flag, outsourcing validator key management
to a remote signer, which validator clients connect to.

Supported by all VCs except Lighthouse through this PR.

Slashing protection DB is not enabled for web3signer since that would
require adding significantly more complexity - creating a separate
postgres DB and running migrations.

I was thinking a lot what to call the remote signer service, in the end
went with `signer-1-geth-lodestar` (this is the remote signer for
`vc-1-geth-lodestar`).

Addresses #263 / #758 

Prerequisite for adding [Vero](https://github.com/serenita-org/vero) as
a validator client - it exclusively uses a remote signer

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Co-authored-by: Barnabas Busa <busa.barnabas@gmail.com>