Register plugin from entry points

[Ni-g-3l]

Hello

As suggest here I implemented the support of entrypoints in order to make the process of adding plugin to REZ easier.

Closes #1661

Feel free to add feedback !

Have a great day

[codecov]

Codecov Report

Attention: Patch coverage is 69.69697% with 10 lines in your changes missing coverage. Please review.

Project coverage is 59.25%. Comparing base (c039654) to head (da9e66c).

Files with missing lines	Patch %	Lines
src/rez/plugin_managers.py	69.69%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1872      +/-   ##
==========================================
- Coverage   59.27%   59.25%   -0.03%     
==========================================
  Files         126      126              
  Lines       17217    17166      -51     
  Branches     3017     2754     -263     
==========================================
- Hits        10206    10171      -35     
+ Misses       6326     6310      -16     
  Partials      685      685              

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