The xcube server STAC API now publishes all fields available via the `/datasets` endpoint.

[konstntokas]

Closes #925

Furthermore, all fields of each dataset published via the /dataset endpoint have been been added to the xcube server STAC API.

Checklist:

• [x] Add unit tests and/or doctests in docstrings
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [ ] ~New/modified features documented in docs/source/*~
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)