refactor: simplify `EthRpcClient`

[gregorydemay]

Follow-up on #275 to simplify EthRpcClient:

1. simplify instantiation by removing intermediate methods between CandidRpcClient::new (the caller) and EthRpcClient::new
2. Change the field EthRpcClient::providers to be non-optional, since any instantiation of the client checks that the given providers are not empty.
3. Remove the trait RpcTransport and the dependency on async-trait, since a single implementation was used