enhancement(sharing): Return newly created driveItem

When accepting a share via 'POST /v1beta1/drives/{driveId}/root/children'
return the newly created driveItem. This driveItem wraps the accepted
remoteItem representing the shared resource (similar to the
'sharedWithMe' response.

This also refactors some of the helpers for user lookup and CS3 share to
driveItem conversion so they can be more easily shared.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f