Implement Remote Signer gRPC Server

[rauljordan]

This is a tracking issue for implementing a gRPC server for the following protobuf service definition: prysm remote keymanager v2 PR. Namely, the server needs to implement two main RPC methods: ListAccounts and Sign. Aside from that, it can choose to store its secrets however it wants, with possible integration of a third party such as Hashicorp Vault. This server should only accept secure gRPC connections.

• Create the gRPC server with secure options
• Implement account generation
• Decide how to store secrets: (possibly simple direct keystores for now, with hashicorp integration in the future)
• Implement the sign method
• Add comprehensive README
• Keep track of data that was previously signed to prevent slashable conditions (can be naively implemented for now)
• Add gRPC gateway support

This implementation is meant to be a reference approach, and not likely something used extensively in production. Notwithstanding, it should have sufficient tests and top-notch documentation for anyone building a remote signer for Prysm.

[terencechain]

Working on Create the gRPC server with secure options