Extract minimal ethereum client (#359)

* relay-ethereum-client

* use relay-ethereum-client from ethereum-poa-relay

* cargo fmt --all

* #![warn(missing_docs)]

* EthereumRpcClient -> EthereumClient

* make EthereumHeadersSyncPipeline private

* return concrete type from crate::new

* cleanup dependencies

* *self -> self

* remove trait Client

* sort deps