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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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