Add support for checking pull request assignees

[jonathanio]

Add the initial code to check for any assignees on the pull request, and add the owner of the pull request to the assignees if none have been set.

Part Of: #1

Checklist

Please confirm the following checks:

• My pull request follows the guidelines set out in CONTRIBUTING.md.
• I have performed a self-review of my code and run any tests locally to check.
• I have added tests that prove my changes are effective and work correctly.
• I have made corresponding changes to the documentation as needed.
• I have checked my code and corrected any misspellings.
• Each commit in this pull request has a meaningful subject & body for context.
• I have squashed all "fix(up)" commits to provide a clean code history.
• My pull request has an appropriate title and description for context.
• I have linked this pull request to other issues or pull requests as needed.
• I have added type/..., changes/..., and 'release/...' labels as needed.

[codecov]

Codecov Report

Merging #21 (cd10e3c) into main (0dec776) will increase coverage by 1.63%.
The diff coverage is 44.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   46.19%   47.82%   +1.63%     
==========================================
  Files           5        6       +1     
  Lines         184      230      +46     
==========================================
+ Hits           85      110      +25     
- Misses         99      119      +20     
- Partials        0        1       +1     

Impacted Files	Coverage Δ
internal/action/main.go	0.00% <0.00%> (ø)
internal/github/main.go	0.00% <0.00%> (ø)
internal/action/check_assignee.go	89.28% <89.28%> (ø)