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.
In this commit, we add the ComputeInputScript which will allow callers
to obtain witnesses for all outputs under control of the wallet. This
allows external scripting of things like coin join, etc.
