[SDESK-7442] Migrate Events resource service to async

[eos87]

Purpose

Extend the new Event async resource service with the next methods

• on_create
• create
• on_created
• validate_event and related private methods
• on_update
• update
• on_updated
• on_deleted

What has changed

• The methods mentioned above have been migrated from old sync service to new async one and the corresponding code has been adjusted as much as possible to be async (along with the depending internal/private methods)
• Added new events_utils module in order to make some of the functions reusable
• Adjusted related test to run

What's pending

• Move all the tests that have references to get_resource_service("events"). There was not enough time to do so, although some tests were already adjusted and fixed.
• Some tests are broken because of an issue with prod_api on superdesk-core (AttributeError: __enter__) produced here
  https://github.com/superdesk/superdesk-core/blob/88d2543a841896731725cb5f2e5cd2b7b34d2a6a/prod_api/conftest.py#L53-L55
  (most likely because prodapi_app.app_context() is not awaited)

Note

I will go through the docstrings of these changes and update them. If there is not time before sprint end, I will provide another small PR for that.

Thanks for reviewing!

Resolves: SDESK-7442

[MarkLark86]

Looks good, just a couple small suggestions

[BrianMwangi21]

LGTM!

[eos87]

I will fix the test in a subsequent PR.