crypto: Log content of received m.room_key_withheld messages

[richvdh]

Log the content of received m.room_key_withheld to-device messages, to fix #3524.

Resulting logging looks like:

12:41:27.593 DEBUG matrix_sdk_crypto::machine: Received a to-device event
    at /home/rav/work/matrix-rust-sdk/crates/matrix-sdk-crypto/src/machine.rs:1194
    in matrix_sdk_crypto::machine::receive_to_device_event with sender="@t:xps9320.sw1v.org" event_type="m.room_key.withheld" 
    in matrix_sdk_crypto::machine::receive_sync_changes
12:41:27.594 DEBUG matrix_sdk_crypto::machine: Processing `m.room_key.withheld` event
    event.content=MegolmV1AesSha2(Unverified(AnyWithheldContent { room_id: "!vFUWYRWcpYVFEHwNMQ:xps9320.sw1v.org", session_id: "yVEWjMz5X2dEsPYmZq1MGM3Fck6fuw4eglv/ARSsbL0", sender_key: "curve25519:O3DORf8A5YJCgxYVwpkKRu3OWd99t9Pn4+7cQjz2CXQ", from_device: Some("KSNXCKMOPV"), .. }))
    at /home/rav/work/matrix-rust-sdk/crates/matrix-sdk-crypto/src/machine.rs:870
    in matrix_sdk_crypto::machine::receive_to_device_event with sender="@t:xps9320.sw1v.org" event_type="m.room_key.withheld" 
    in matrix_sdk_crypto::machine::receive_sync_changes

---

Original description, now outdated:

This is a first attempt at fixing #3524. Obviously, it solves the reported problem, however there is a concern it might be too spammy.

Otherwise, we might have to be more selective here... maybe only log withheld events?

It shouldn't lead to logging any extra sensitive data - this is all stuff that is shared with the homeserver anyway.

[richvdh]

Resulting logging looks like:

 DEBUG matrix_sdk_crypto::machine: Received a to-device event
    event=RoomKeyWithheld(ToDeviceEvent { sender: "@t:xps9320.sw1v.org", content: MegolmV1AesSha2(Unverified(AnyWithheldContent { room_id: "!vFUWYRWcpYVFEHwNMQ:xps9320.sw1v.org", session_id: "Z77mee5wEtJFA4FBuMZ5s0OSUFmYQhJ1F5vdEVAfSEI", sender_key: "curve25519:O3DORf8A5YJCgxYVwpkKRu3OWd99t9Pn4+7cQjz2CXQ", from_device: Some("KSNXCKMOPV"), .. })), other: {} })
    at /home/rav/work/matrix-rust-sdk/crates/matrix-sdk-crypto/src/machine.rs:1193
    in matrix_sdk_crypto::machine::receive_to_device_event with sender="@t:xps9320.sw1v.org" event_type="m.room_key.withheld" 
    in matrix_sdk_crypto::machine::receive_sync_changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.83%. Comparing base (e89659b) to head (5433a94).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3591   +/-   ##
=======================================
  Coverage   83.83%   83.83%           
=======================================
  Files         254      254           
  Lines       26167    26168    +1     
=======================================
+ Hits        21936    21937    +1     
  Misses       4231     4231           

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[poljar]

Let's not log contents of every event we receive.