This commit adds the ability to generate liboverrides of linked data at
the `BKE_blendfile_link_append` BKE level, and through the Python API
(the `BPY_library_load` context manager, aka `bpy.data.libraries.load`).
The python API was updated essentially to allow easy testing of the new
code. This commit also adds tests for the new 'override' behavior, and
for existing basic link one.
Current code only generates 'basic' overrides, without any handing of
hierarchies or dependencies, as for brush assets only the Brush ID needs
to be overridden.
That new feature does not aim at being exposed to user through the
link/append operations in its current state, as it is way too simplistic.
This change is a requirement for the Brush Asset project (#101908).
Bastien Montagne