Allow for protocol to be specified in eth2 client connection

[moles1]

The eth2 provider URL in config.yml does not specify a protocol, since the clients use different protos to communicate (e.g. http for lighthouse, gRPC for Prysm). The Lighthouse, Nimbus and Teku API clients are all hard-coded to use HTTP. This should be made configurable so that they can communicate over HTTPS to support services like Infura.