Apps implementing OCM endpoints via OCMEndpointRequestEvent (e.g.
SUNET/nextcloud-ocm_request_share for request-share, nextcloud/contacts
for invite-accepted) need to apply the same identity check that the
built-in addShare and receiveNotification handlers apply, so it makes
sense to make it publicly accessible.
It also allows us to refactor RequestHandlerController::confirmSignedOrigin
to use the new public method and drop the confirmNotificationIdentity helper.
Signed-off-by: Micke Nordin <kano@sunet.se>
