lnrpc/walletrpc: add new sub-RPC server, the WalletKit

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.

lnrpc/signrpc/signer.pb.go

@@ -376,6 +376,7 @@ func (m *InputScript) GetSigScript() []byte {
 }
 
 type InputScriptResp struct {
+	// / The set of fully valid input scripts requested.
 	InputScripts []*InputScript `protobuf:"bytes,1,rep,name=input_scripts,json=inputScripts" json:"input_scripts,omitempty"`
 }