Support dynamic Extension Repository paths via BLENDER_SYSTEM_SCRIPTS

Hello!

Thanks to work from Sony Interactive Entertainment, we have received support for multiple paths in BLENDER_SYSTEM_SCRIPTS , which enabled us to read addons, startup scripts and other things automatically from multiple locations. projects.blender.org/blender/blender/pulls/132812

This is highly needed in studio environments to support packaged application environments. There is some good discussion in here (with a great comment by Matt Ebb), which does a good job at explaining the reasons: devtalk.blender.org/t/blender-user-scripts-and-multiple-paths/10395/5

Unfortunately, extension repositories (and their paths) have to be explicitly added + managed and get saved in the preferences. This means that if one wants to gather extensions from a multitude of packages, one will need to both remove the previously added local repositories and add the now needed ones. This is unfortunately quite tricky and could be improved.

My proposal would be to add support for local extension repositories to be picked up automatically via subpaths in BLENDER_SYSTEM_SCRIPTS.

For example this path could be like the following:

{BLENDER_SYSTEM_SCRIPTS}
└── extensions
    └── system

(with context of the other usual Blender folders)

{BLENDER_SYSTEM_SCRIPTS}
├── config
├── scripts
│   ├── startup
│   └── addons
└── extensions
    └── system

In this case Blender could detect the {BLENDER_SYSTEM_SCRIPTS}/extensions/system path and automatically add it to the local repositories.

Personally, I don't have any strong opinion about the concrete path, but for studios it would be very much needed to get some help to dynamically manage extension repositories.

Thank you,
Manuel