Add Webdav property to advise the sync clients to not sync this entry

[PVince81]

There are several tickets that propose different approaches and different criteria for sync exclusions. Regardless of the criteria, I think the basic mechanism should be that the server returns an extra Webdav property <oc:do-not-sync>1</oc:do-not-sync> to tell the client to not sync this item.

It is then the responsibility of apps or whichever code that decides what the exclusion criteria is to set the property on the entries.

This is purely the backend/protocol level.

Some proposed criteria that would need to be implemented separately:

• excluding external storage: https://github.com/owncloud/core/issues/7234
• exclusion by user/group: https://github.com/owncloud/core/issues/25873
• pattern-based, similar to sync-exclude.lst: https://github.com/owncloud/core/issues/19385
• exclusion by patterns: https://github.com/owncloud/core/issues/22607
• "certain directories": https://github.com/owncloud/core/issues/7909
• excluding federated share individually from sync: https://github.com/owncloud/core/issues/14727
• excluding individual ext storages from syncing: https://github.com/owncloud/core/issues/12216 (see sync option)

@owncloud/filesystem @guruz @ogoffart @dragotin @davivel @DeepDiver1975 @butonic