Support providing stability for xattr components

[jlebon]

Basically fix this TODO:

chunkah/src/components/xattr.rs

Line 141 in 8031677

// TODO: make this configurable via xattr or CLI

This is actually trivial work but the UX needs some thought. Stability is a component-level value, but the main way users interact with the xattr component is by labeling files.

Perhaps simplest is to just support a e.g. user.stability value on any of the files in that component and error out if there are multiple values which disagree.

[jlebon]

And, perhaps we should flip the default stability of xattr objects when none is given from the min stability heuristic we have to e.g. the median? Essentially the act of users labeling files with xattrs is itself a signal that they think these components have some level of stability. So minimum stability doesn't make sense.