Support FS-specific attributes / flags

This is a meta-issue pointing to some similar other ones (so that we can reduce the number of open issues).

borg supports a few filesystem flags (aka bsdflags), esp. the more popular ones and the ones present on most platforms.

Beyond that, some filesystems have special attributes / flags:
- #2805
- #1557
- #618

It is unclear whether borg should support such fs-specific flags / attributes.

This is not just about adding some metadata to the archived item, but also:
- borg would need to detect the fs to know which attributes / flags it can extract. it might even depend on the version of the filesystem...
- would need fs-specific code in borg (we try to avoid this)
- cross-filesystem differences (would borg try to map flags between filesystems?)
- it is sometimes not easy to deal with those flags, especially the time when they need to get set or how they influence other file metadata (e.g. timestamps)
- ...