Log the session ID in m.room_key.withheld

[kegsay]

It makes debugging easier. Currently they get logged like this:

2024-06-05T16:17:36.266466Z DEBUG matrix_sdk_crypto::machine: Received a to-device event | crates/matrix-sdk-crypto/src/machine.rs:1191 | spans: root > next_sync_with_lock{store_generation=8226 store_generation=8226} > sync_once{pos="2" pos="2"} > handle_response > preprocess_to_device_events > receive_sync_changes > receive_to_device_event{sender="@xxx:element.io" sender="@xxx:element.io" event_type="m.room_key.withheld" event_type="m.room_key.withheld" message_id="e380dcf0-571d-4271-818e-e556bb041f4d" message_id="e380dcf0-571d-4271-818e-e556bb041f4d"}

This isn't enough to tie it to the session ID without also having the sender rageshake (as that includes the message_id).