lnrpc/watchtowerrpc: add WatchtowerBackend configs

lnrpc/watchtowerrpc/config_active.go

@@ -0,0 +1,13 @@
+// +build watchtowerrpc
+
+package watchtowerrpc
+
+// Config is the primary configuration struct for the watchtower RPC server. It
+// contains all items required for the RPC server to carry out its duties. The
+// fields with struct tags are meant to parsed as normal configuration options,
+// while if able to be populated, the latter fields MUST also be specified.
+type Config struct {
+	// Tower is the active watchtower which serves as the primary source for
+	// information presented via RPC.
+	Tower WatchtowerBackend
+}