Allow mapping google.protobuf.duration to Optional datetime.timedelta

[malteneuss]

We had some issues loosing a zero-duration. Currently, if a serialized Protobuf message contains a 0-duration like myDuration in this example:

syntax = "proto3";

message Some {
  string someField = 1;
  google.protobuf.duration myDuration = 2;
}

and data like

{
  "someField": "42",
  "myDuration": "0s"         
}

this myDuration field gets correctly deserialized into a timedelta(0). However, when forwarding/serializing that Python object again, the duration gets lost:

{
  "someField": "42",
  // myDuration missing
}

Because timedelta(0) is the default value, it appearently represents no-value in betterProto (see Well known Google Types).

Is it possible to configure the mapping from Protobuf's duration to an Optional timedelta value so that no-value can be distinguished from a 0-value in Python?