proto: generate proto files with go generate

During codebase changes, I had to modify proto files. Noticed a lack
of instructions for compiling them.

This commit adds support for running `go generate ./...` to generate the
pb.go files.

cluster/proto/gen.go

@@ -0,0 +1,2 @@
+//go:generate protoc -I../../ -I. --go_out=. --go_opt=paths=source_relative message.proto
+package proto

cluster/proto/message.pb.go

@@ -1,7 +1,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.32.0
-// 	protoc        v3.6.1
+// 	protoc        v3.12.4
 // source: message.proto
 
 package proto

command/proto/command.pb.go

@@ -1,7 +1,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.32.0
-// 	protoc        v3.6.1
+// 	protoc        v3.12.4
 // source: command.proto
 
 package proto

command/proto/gen.go

@@ -0,0 +1,2 @@
+//go:generate protoc --go_out=. --go_opt=paths=source_relative command.proto
+package proto