Currently, the import nodes always reimport on each evaluation. This patch adds
support for caching the loaded geometries. This is integrated with
`BLI_memory_cache.hh` and thus also takes the cache size limit into account. If
an imported file is modified on disk, the cache is invalidated. However,
Geometry Nodes will not automatically reevaluate when a file changes, so the
user would have to trigger the evaluation in some other way.
This is an alternative solution to #124369. The main benefits are that the cache
invalidation happens automatically and that the cache system is more general and
does not have to know about e.g. the different file types.
Caching speeds up node setups that heavily rely on import nodes significantly.
Pull Request: https://projects.blender.org/blender/blender/pulls/138425
