Switch protobuf generation to buf cli tool (#720)

Current generation of protobuf stubs is hard and there's open issues
around it:
   https://github.com/grpc-ecosystem/grpc-gateway/issues/1065

grpc-gateway author and README suggest to use `buf` instead which
intends to be a simpler and more reliable method of protobuf stub
generation.

gctrpc/buf.lock

@@ -0,0 +1,17 @@
+# Generated by buf. DO NOT EDIT.
+version: v1
+deps:
+  - remote: buf.build
+    owner: beta
+    repository: googleapis
+    branch: main
+    commit: 1c473ad9220a49bca9320f4cc690eba5
+    digest: b1-unlhrcI3tnJd0JEGuOb692LZ_tY_gCGq6mK1bgCn1Pg=
+    create_time: 2021-06-23T20:16:47.788079Z
+  - remote: buf.build
+    owner: grpc-ecosystem
+    repository: grpc-gateway
+    branch: main
+    commit: d19475fa22444a289c46af009acce62c
+    digest: b1-_zhDPyr_Ctc1QRAKuad6_0xvoyPd6QaB22ldm9gzS0Q=
+    create_time: 2021-04-26T15:19:26.742789Z