integrate request builder into HTTP client for googleapis support (#157)

builder/proto/errors.pb.go

@@ -0,0 +1,15 @@
+package proto
+
+import "google.golang.org/protobuf/encoding/protojson"
+
+var marshaler = protojson.MarshalOptions{}
+
+func (m *Test_Client_Call_DefaultError) Error() string {
+	buf, _ := marshaler.Marshal(m)
+	return string(buf)
+}
+
+func (m *Test_Client_Call_SpecialError) Error() string {
+	buf, _ := marshaler.Marshal(m)
+	return string(buf)
+}