Rename the Godot OpenXR Loaders plugin to Godot OpenXR Vendors plugin

[m4gr3d]

This reflects the expansion in roles and capabilities of the plugin.

[kisg]

Quick question: Shouldn't we package the vendor-specific code as a separate plugin for each vendor? E.g. one plugin for meta, one for pico ... etc. This plugin would include the loader and any vendor-specific extension code (like the meta scene reconstruction extension).

We could still keep all these plugins in this single repository.

[m4gr3d]

Quick question: Shouldn't we package the vendor-specific code as a separate plugin for each vendor? E.g. one plugin for meta, one for pico ... etc. This plugin would include the loader and any vendor-specific extension code (like the meta scene reconstruction extension).

We could still keep all these plugins in this single repository.

That's the approach I first considered but it introduces overhead (e.g: updating & maintaining nearly identical separate plugins, loading 4 separate plugins for dev & testing instead of a single one, etc) for no real benefits. With this approach, we can leverage the fact they're all part of the same plugin to share common code, and provide better documentation and warnings to users (e.g: only 1 plugin should be selected for each export preset).

Of course, if a vendor were to step forward with the intent of developing and maintaining their own plugin, we'd let them fork from this repo and create their own plugin supporting only their device(s) (and to avoid confusion, we'd remove that vendor from this plugin).