In this commit, we add a new sub-RPC server to the existing set of gRPC
servers. This new sub-RPC server is the WalletKit. It's a utility
toolkit that contains method which allow clients to perform common
interactions with a wallet such as getting a new address, or sending a
transaction. It also includes some supplementary actions such as fee
estimation.
One thing to note in the RPC file is that we _import_ the existing
signer.proto file in order to get at some existing proto definitions
which are useful in our use case.
