Having DavObject::etag property writable about cdav-library

[skjnldsv]

Of course :)
Example:

1. I open a tab with contacts and click the first contact
2. I open the another one and edit the same contact with a random change
3. Back on the first tab, edit the contact
4. A popup will appear stating a change have been detected on the server and you need to click a button to resync the updated contact and that what you did will be discarded (this is a nice ux behaviour, no data loss, clear explanation)
5. we retrieved the etag on the last sync attempt (denied since etag obsolete) so we can manually update the cdav-contact object to specify the new etag manually and trigger a new fullFetch to update everything :)

[skjnldsv]

@georgehrke is it clear? 🙈
If you need a precise example, this is exactly here:
https://github.com/nextcloud/contacts/blob/134527baa933af63400c285b362d9ed889abb001/src/store/contacts.js#L281
We then use this conflict saved etag to update the dav scope linked to the vcard and force a ful refresh :)