[rai] Fix extraction for recent raiplay.it updates

[rai] Fix extraction for recent raiplay.it updates

- Introduce _BASE_URL in RaiBaseIE class so it could be reused as base for the
  several subextractors.
- Remove first test of RaiPlayIE, it is no longer available
- Adjust RaiPlayIE to recent raiplay.it updates, make it extension agnostic
  (passing possible `.json' URLs is now supported too) and update test
  info_dict.
- Adjust RaiPlayLiveIE to recent raiplay.it updates.  Passing it as
  `url_transparent' is no longer supported (there is no longer an accessible
  ContentItem)
- Adjust RaiPlayPlaylistIE to recent raiplay.it updates and instruct it about
  ContentSet-s.
- Update a RaiIE test and remove two tests that are no longer availables

This fix issue #22923, #22906 and supersedes #23006.