Blender Git Loki

Assets: add global `bke::AssetLibraryService` class

Add `blender::bke::AssetLibraryService` class that acts like a
blendfile-scoped singleton. It's allocated upon the first call to
`BKE_asset_library_load` and destroyed in the LOAD-PRE handler.

The `AssetLibraryService` ensures that edits to asset catalogs are not
lost when the asset browser editor closes (or even reloads). Instead,
the `AssetLibrary` pointers it owns are kept around as long as the blend
file is open.

Reviewed By: Severin

Maniphest Tasks: T92151

Differential Revision: https://developer.blender.org/D12885