Duration well known type not supported

[delaneyj]

In feature it says

supports all well-known-types with custom JSON representation and helper methods

but google.protobuf.Duration is not supported. It has the same structure as a Timestamp but different contextual meaning.

[timostamm]

Take a look at how Duration serializes to JSON - it's very different from `Timestamp'. Closing this because "not supported" does not seem accurate - unless a regression somehow slipped by the tests. Please LMK if you are aware of something in that regard.

If you have some ideas for helper functions specific to Duration, I'm all ears (we don't really have any standard except Date, so I've found that a bit difficult without pulling in the kitchen sink). Note that the plan is to switch to the base types of protobuf-es, so the preferable way forward would be to add helpers there.