Improving RPC method for getting list of JSON from CAWS

[bardram]

Right now, when CAWS is returning a list of items (like invitations), the RPC method converts this into a JSON map with just one item, called items.

https://github.com/cph-cachet/carp.sensing-flutter/blob/a2c21b05a2cb8e921767e4c78d2cc973e8ca9fc0/backends/carp_webservices/lib/carp_services/carp_base_service.dart#L131-L182

This is a hack and should be handled better, similar to the unit test in carp_core:

https://github.com/cph-cachet/carp.sensing-flutter/blob/a2c21b05a2cb8e921767e4c78d2cc973e8ca9fc0/carp_core/test/carp_core_dart_test.dart#L143-L161