Use the new JSONProvider mechanism in recent flask/quart

Recent versions of Flask & Quart no longer support the app.json_encoder mechanism that we have been using to handle JSON serialization of our Pydantic model classes, instead they have moved to a pluggable JSONProvider class rather than separate encoders and decoders.

Note that this MR pushes the minimum supported Flask and Quart versions to 2.2 and 0.18.0 respectively