Plugin system

[JeanChristopheMorinPerso]

Draft PR for visibility. It kind of works on Linux.

It adds a new plugin system with multiple hooks. There is an example plugin to add support for correctly installing PySide6>=6.3. The plugin takes care of merging all the PySide6 related packages (PySide6, PySide6-Addons and PySide6-Essentials) into one rez package. There is also a shiboken6 plugin that takes care of removing the vendored PySide6 stuff...

$ rez-pip2 PySide6 --prefix /tmp/asd --python 3.11 -- --find-links /tmp/wheels --no-index

$ rez-env PySide6 --paths ~/rez_packages:/tmp/asd -- python -c 'import PySide6,shiboken6; print(PySide6); print(shiboken6)'
<module 'PySide6' from '/tmp/asd/PySide6/6.6.1/9645a50b415bcaad6bcde791aeb0859a43c56501/python/PySide6/__init__.py'>
<module 'shiboken6' from '/tmp/asd/shiboken6/6.6.1/9645a50b415bcaad6bcde791aeb0859a43c56501/python/shiboken6/__init__.py'>

:tada:

TODOs:

• Add option to disable plugins/hooks
• Test if builtin plugins can be overridden
• Test on Windows
• Write tests
• Cleanup the code
• Write docs (and automatically generate docs for builtin plugins).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 67.32283% with 83 lines in your changes are missing coverage. Please review.

Project coverage is 77.46%. Comparing base (f3bd30f) to head (361601a).

Files	Patch %	Lines
src/rez_pip/plugins/PySide6.py	50.90%	19 Missing and 8 partials :warning:
src/rez_pip/cli.py	26.92%	16 Missing and 3 partials :warning:
src/rez_pip/pip.py	69.69%	9 Missing and 1 partial :warning:
src/rez_pip/plugins/__init__.py	87.27%	5 Missing and 2 partials :warning:
src/rez_pip/plugins/shiboken6.py	58.82%	5 Missing and 2 partials :warning:
src/rez_pip/install.py	50.00%	4 Missing :warning:
src/rez_pip/rez.py	87.50%	1 Missing and 3 partials :warning:
src/rez_pip/utils.py	25.00%	2 Missing and 1 partial :warning:
src/rez_pip/compat.py	75.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #91 +/- ## ========================================== - Coverage 81.25% 77.46% -3.79% ========================================== Files 8 12 +4 Lines 720 914 +194 Branches 150 207 +57 ========================================== + Hits 585 708 +123 - Misses 119 173 +54 - Partials 16 33 +17 ```

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