lnrpc/walletrpc: implement the WalletKitServer gRPC service

In this commit, we implement the newly defiend WalletKitServer gRPC
service. We use the same template w.r.t build tags as the existing
signrpc service.

lnrpc/walletrpc/config_active.go

@@ -0,0 +1,41 @@
+// +build walletrpc
+
+package walletrpc
+
+import (
+	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/macaroons"
+)
+
+// Config is the primary configuration struct for the WalletKit RPC server. It
+// contains all the items required for the signer rpc server to carry out its
+// duties. The fields with struct tags are meant to be parsed as normal
+// configuration options, while if able to be populated, the latter fields MUST
+// also be specified.
+type Config struct {
+	// WalletKitMacPath is the path for the signer macaroon. If unspecified
+	// then we assume that the macaroon will be found under the network
+	// directory, named DefaultWalletKitMacFilename.
+	WalletKitMacPath string `long:"walletkitmacaroonpath" description:"Path to the wallet kit macaroon"`
+
+	// NetworkDir is the main network directory wherein the signer rpc
+	// server will find the macaroon named DefaultWalletKitMacFilename.
+	NetworkDir string
+
+	// MacService is the main macaroon service that we'll use to handle
+	// authentication for the signer rpc server.
+	MacService *macaroons.Service
+
+	// FeeEstimator is an instance of the primary fee estimator instance
+	// the WalletKit will use to respond to fee estimation requests.
+	FeeEstimator lnwallet.FeeEstimator
+
+	// Wallet is the primary wallet that the WalletKit will use to proxy
+	// any relevant requests to.
+	Wallet lnwallet.WalletController
+
+	// KeyRing is an interface that the WalletKit will use to derive any
+	// keys due to incoming client requests.
+	KeyRing keychain.KeyRing
+}