Suport relative path option per-asset library

This option is true by default, but it can be changed for
any asset library (that may be using Link as import method).

This also fix "Reset to Default Value" for the Import Method
since this was originally not using the defaults.

Pull Request: https://projects.blender.org/blender/blender/pulls/107345

scripts/startup/bl_ui/space_userpref.py

@@ -1491,6 +1491,7 @@ class USERPREF_PT_file_paths_asset_libraries(FilePathsPanel, Panel):
         active_library = paths.asset_libraries[active_library_index]
         layout.prop(active_library, "path")
         layout.prop(active_library, "import_method", text="Import Method")
+        layout.prop(active_library, "use_relative_path")
 
 
 class USERPREF_UL_asset_libraries(bpy.types.UIList):