Blender Git Loki

Assets: Recursive reading of asset libraries

With this, asset libraries can be directory structures and all assets in
sub-directories will show up in an Asset Browser.

With complex directory structures and many .blend files inside, asset
library reading will be quite slow for now. There's initial work being
done to introduce indexing for this (T82979), other optimizations are
being discussed as well.

Addresses T91406.

Differential Revision: https://developer.blender.org/D12139