Add PREMIS events

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.30939% with 93 lines in your changes missing coverage. Please review.

Project coverage is 66.77%. Comparing base (f976317) to head (84e8e73).

Files	Patch %	Lines
internal/premis/premis.go	77.29%	34 Missing and 13 partials :warning:
cmd/worker/workercmd/cmd.go	0.00%	12 Missing :warning:
internal/workflow/preprocessing.go	84.37%	5 Missing and 5 partials :warning:
internal/activities/add_premis_objects.go	66.66%	4 Missing and 4 partials :warning:
internal/activities/add_premis_event.go	62.50%	3 Missing and 3 partials :warning:
internal/activities/validate_file_formats.go	68.42%	3 Missing and 3 partials :warning:
internal/activities/add_premis_agent.go	69.23%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #24 +/- ## ========================================== + Coverage 62.79% 66.77% +3.97% ========================================== Files 14 19 +5 Lines 586 936 +350 ========================================== + Hits 368 625 +257 - Misses 198 261 +63 - Partials 20 50 +30 ```

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[mcantelon]

Thanks @jraddaoui ! I've addressed most of the feedback, but will fix the rest, add more tests, and add PREMIS event appending from within activities.

[djjuhasz]

@mcantelon I just realized I may not have been clear that passing around io.Reader, io.Writer, and *etree.Document should only be done inside of an activity. We can't return large data structures like *etree.Document from an activity, so each PREMIS activity should read the existing premis.xml from disk (if it exists) at the beginning of the activity, and write a premis.xml file to disk at the end of the activity.

[mcantelon]

Thanks @djjuhasz... should be good for another review!