fsspec-cache-clear: add config variables for clearing cache up

[yarikoptic]

ATM the entire cache is cleaned up by the command.

• Analogously to https://github.com/joblib/joblib/pull/1200/files we need here to allow for age (or atime_age, as in "last accessed", not when was created), and size limits, so we could keep caches "under control".
  • I think those should be DataLad configuration options. Then they could be made universally applicable (through centralized configuration) or specified on command line (or in Python interface, although there not as easily ATM iirc). So let's make them
  • I think we do not have formalized way to extend the dictionary of known config variables (filed formalize extending common_cfg.definitions (and other things) in extensions? datalad#6314) so we can simply use/document such config variables in this extension: datalad.fsspec.cache-clear.age and datalad.fsspec.cache-clear.size which would default to None (not considered)
• Ideally we should have also allowed for an option only_in_file_tree to remove cached items which are no longer available in the current git (annex) file tree (of current commit). But 1). I do not think it would be easily possible without adding some extra layer of tracking; 2). such items would eventually could be picked up through age. So, let's not bother about this one

[jwodder]

@yarikoptic While fsspec has the necessary pieces to implement such a cleanup in datalad-fuse, the code would end up relying largely on undocumented implementation details of how fsspec structures its cache metadata. Are you sure that's the best idea?

[yarikoptic]

I agree that we must not implement something based on some undocumented implementation details. I think then an issue (or better a PR) should be filed with fsspec to expose (and documenting) interface(s) to provide such functionality. In the minimalistic case it could be some get_cache_entries function which would return entries within cache with all needed attributes (path, atime, size) so our code then could implement the rest.

[jwodder]

@yarikoptic Issue created: fsspec/filesystem_spec#857