Allow proto tag to define field name

When we introduce a new field for backwards compatibility, we may want
to specify a different protobuf field name (one that matches JSON) than
the automatic transformation applied to the struct field. This allows an
API field to define the name of its protobuf tag.

cmd/libs/go2idl/go-to-protobuf/protobuf/generator.go

@@ -589,6 +589,8 @@ func protobufTagToField(tag string, field *protoField, m types.Member, t *types.
 			return fmt.Errorf("member %q of %q malformed 'protobuf' tag, tag %d should be key=value, got %q\n", m.Name, t.Name, i+4, extra)
 		}
 		switch parts[0] {
+		case "name":
+			protoExtra[parts[0]] = parts[1]
 		case "casttype", "castkey", "castvalue":
 			parts[0] = fmt.Sprintf("(gogoproto.%s)", parts[0])
 			protoExtra[parts[0]] = parts[1]