Register plugins 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

❌ Patch coverage is 67.27273% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.09%. Comparing base (cd8476f) to head (a24233d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/plugin_managers.py	68.51%	12 Missing and 5 partials ⚠️
src/rez/rezconfig.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1991      +/-   ##
==========================================
+ Coverage   60.01%   60.09%   +0.07%     
==========================================
  Files         163      163              
  Lines       20098    20131      +33     
  Branches     3494     3499       +5     
==========================================
+ Hits        12062    12097      +35     
+ Misses       7224     7221       -3     
- Partials      812      813       +1     

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[JeanChristopheMorinPerso]

Thanks @Ni-g-3l . Sorry if it took me so long to come back to this PR. I'll do some more tweaks to the docs and I think it'll be ready to go.

[JeanChristopheMorinPerso]

I am happy with the result. Thanks a lot @Ni-g-3l for your contribution!