Load plugins from entry points

[abkfenris]

Enables the loading of plugins via entry points. This should enable libraries to install and advertise their own plugins without requiring as much user configuration.

Entry points are also used for automatically generating rule documentation.

Closes #69

[abkfenris]

Currently a little entangled in my ACDD test PR #65 which is keeping me from removing the now auto-generated rule ref doc and changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.48%. Comparing base (353e655) to head (79ae08a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #70   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files          74       74           
  Lines        2927     2935    +8     
  Branches      425      426    +1     
=======================================
+ Hits         2912     2920    +8     
  Partials       15       15           

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