lnrpc: split REST annotations into service files

As a preparation for the migration to the grpc-gateway/v2 library we
declare each service's REST annotations in its own file. This is
optional in the v1 library but mandatory in v2.

lnrpc/signrpc/signer.yaml

@@ -0,0 +1,20 @@
+type: google.api.Service
+config_version: 3
+
+http:
+  rules:
+    - selector: signrpc.Signer.SignOutputRaw
+      post: "/v2/signer/signraw"
+      body: "*"
+    - selector: signrpc.Signer.ComputeInputScript
+      post: "/v2/signer/inputscript"
+      body: "*"
+    - selector: signrpc.Signer.SignMessage
+      post: "/v2/signer/signmessage"
+      body: "*"
+    - selector: signrpc.Signer.VerifyMessage
+      post: "/v2/signer/verifymessage"
+      body: "*"
+    - selector: signrpc.Signer.DeriveSharedKey
+      post: "/v2/signer/sharedkey"
+      body: "*"