Add unit tests for `package_.CreatePreservationTask()`

[djjuhasz]

Fixes #900

• Split large files related to PreservationAction and PreservationTask entities into smaller files
• Add a persistence.CreatePreservationTask() method, and add an implementation to the ent client
• Call persistence.CreatePreservationTask() from package_.CreatePreservationTask() instead of using direct SQL queries
• Add unit tests
• Remove the persistence.eventManager service wrapper
• Update the ent schema for the "preservation_action" and "preservation_task" tables to make the timestamps columns nullable

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.99363% with 179 lines in your changes are missing coverage. Please review.

Project coverage is 48.00%. Comparing base (f075824) to head (4420aa2).

:exclamation: Current head 4420aa2 differs from pull request most recent head 049f105. Consider uploading reports for the commit 049f105 to get more accurate results

Files	Patch %	Lines
internal/enums/preservation_action.go	0.00%	66 Missing :warning:
internal/package_/preservation_task.go	16.66%	45 Missing :warning:
internal/enums/preservation_task.go	0.00%	37 Missing :warning:
internal/persistence/ent/client/package.go	86.88%	4 Missing and 4 partials :warning:
internal/workflow/local_activities.go	0.00%	8 Missing :warning:
internal/a3m/a3m.go	0.00%	6 Missing :warning:
internal/package_/preservation_action.go	0.00%	4 Missing :warning:
...ternal/persistence/ent/client/preservation_task.go	92.59%	1 Missing and 1 partial :warning:
internal/workflow/processing.go	88.23%	1 Missing and 1 partial :warning:
internal/workflow/move.go	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #901 +/- ## ========================================== + Coverage 47.11% 48.00% +0.88% ========================================== Files 96 101 +5 Lines 5383 5402 +19 ========================================== + Hits 2536 2593 +57 + Misses 2611 2571 -40 - Partials 236 238 +2 ```

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