Add PREMIS-related activities

[mcantelon]

Added activities to add objects, events, and agents to a PREMIS file.

[codecov]

Codecov Report

❌ Patch coverage is 77.53846% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.66%. Comparing base (b0b1966) to head (db34479).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
premis/premis.go	83.26%	30 Missing and 13 partials ⚠️
premis/objects/activity.go	50.00%	12 Missing and 6 partials ⚠️
premis/agent/activity.go	62.50%	4 Missing and 2 partials ⚠️
premis/event/activity.go	62.50%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
- Coverage   84.04%   81.66%   -2.38%     
==========================================
  Files          18       22       +4     
  Lines         564      889     +325     
==========================================
+ Hits          474      726     +252     
- Misses         63      113      +50     
- Partials       27       50      +23     

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