Vfs: Add 'availability', a simplified, user-facing pin state #7111

Vfs: Add 'availability', a simplified, user-facing pin state #7111

The idea is that the user's question is "is this folder's data available
offline?" and not "does this folder have AlwaysLocal pin state?".
The the answers to the two questions can differ: an always-local
folder can have subitems that are not always-local and are dehydrated.

The new availability enum intends to describe the answer to the user's
actual question and can be derived from pin states. If pin states aren't
stored in the database the way of calculating availability will depend
on the vfs plugin.