Add support for overriding non-selected Khronos vendor options

[m4gr3d]

This PR is broken into two commits:

• The first commit cleans up some of the validation logic and ensures that a vendor plugin is enabled prior to executing logic against it (e.g: showing warnings)
• The second commit showcases how we can support multiple vendors under the Khronos loader while keeping a clean user interface. To do so, it makes use of the _get_export_options_overrides(...) method introduced in Godot 4.3. An alternative would be to use _get_export_option_visibility(...) instead, but that method is not yet available to the EditorExportPlugin.
  • Note that the approach used in the second commit is up for discussion, and may not be merged in if we decide to go a different route.

[BastiaanOlij]

This indeed works well, though I'm guessing that once _get_export_option_visibility becomes possible that would be a better solution.

It would also be nice that if the Khronos option isn't selected, all entries related to Khronos aren't shown. The same with the other selections. But that is a nice to have.

I'm happy if we merge this as is with a note that we improve upon this when we can. I'll rebase my MagicLeap PR and add the needed changes in.

[m4gr3d]

I think we should merge this as it is now, and then improve it over time.

Sounds good!

@dsnopek Any thoughts on this approach?