Make provider events a proper irpc protocol

[rklaehn]

Provider events are currently just a tokio mpsc channel of events, where the events sometimes contain oneshot senders for the response. This was done this way because the irpc channel was a spsc channel.

Now that the irpc channel is mpsc, we might change this into a proper irpc protocol. The advantage would be that you could have the logic for access control on a different machine than the blobs provider itself, and also that it would be more uniform.

To save overhead for cases where you don't want fine-grained permissions, you would have a mask in the EventSender that controls whether events are being sent at all.