[Placeholder] Integrate "accept user shares"

[SamuAlfageme]

After owncloud/core#30106 gets merged we should integrate this new flow into the client.

Tagging this as "placeholder" since I'm not familiar just yet neither with the new OCS endpoints nor the full share state machine. I guess if this flow gets wrapped exactly like federated share notifications, there should not be a lot of friction. We'll just need a way to revisit rejected shares if the user reconsiders in the future (by judging what I see on owncloud/core#30106 (comment))

cc/ @PVince81

[SamuAlfageme]

I've been playing around a bit with owncloud/core#30106 and have some concerns still.

I guess if this flow gets wrapped exactly like federated share notifications, there should not be a lot of friction.

Since the notification app is unbundled from core itself (or it can be disabled by those means), one would expect to have a primary "accept shares" mechanism on the client(s) not to rely on apps that might not be present on the server.

@michaelstingl @ckamm what do you think if I throw you guys a demo of the workflow and present the state machine next week so we check different UX approaches to integrate this? We could involve PM and the mobile team as well.