eTag for metadata changes

Updated version:

Provide an eTag for metadata changes (favorite, share, note, etc).
Currently clients (Android / iOS) check for changed eTag when entering a folder.
If this is the same with stored database no real propfind to list folder content is needed.
However if e.g. a new sharee was added, we still need to retrieve it. Therefore we check every time(!)
/ocs/v2.php/apps/files_sharing/api/v1/shares

This has multiple drawbacks

• more server load
• more client load
• UX on client side is not ideal, as sharees are delayed until info is fetched, seeing a spinner

On next android version we wanted to rely on changed eTags when syncing a folder:

• enter a folder
• check remote eTag with locally stored eTag
• refresh content only if eTag differs

The problem now is that with this approach we do not get info about favorited/unfavorited folders. (encrypted/not encrypted I have not checked yet).

In my opinion at least the parent folder should then have a changed eTag to reflect that something has changed.

@skjnldsv @icewind1991

Start with the /ocs/v2.php/apps/files_sharing/api/v1/shares entry point and the existing eTag behavior used when entering or syncing a folder. Define how favorite, share, and note changes affect the parent folder eTag, while still allowing new sharees to be detected. Done means clients can avoid an unnecessary content refresh when the eTag is unchanged and observe the relevant metadata changes when it differs.