Add support for automatically encoding testing.Relation data to JSON

[tonyandrewmeyer]

It's already common practice to have all the field values in relation data be JSON-encoded, and this will increase more once ops supports using data classes to load/save relation data, where the default is to JSON encode/decode each field's value.

Rather than having a lot of json.dumps() in setting up testing.Relation objects, and a lot of json.loads() in asserts, it would be convenient if there was a way to have this happen transparently.

[benhoyt]

Makes sense to me. testing.Relation(..., json=True) maybe?

[benhoyt]

We're not 100% sure about the API, but something like json=True mentioned above.

[dimaqq]

Some charm unit tests use idioms like this:

      tracing = Relation(
          "tracing",
          remote_app_data=TracingRequirerAppData(receivers=[requested_protocol]).dump(),
      )

That being said, manual json.dumps() and manual plain (without json) are used in the same set of unit tests.