[identity] Implement transactional device list updates

Summary:
One of the most important commits in this stack. Added code that allows us to handle transactional device list updates. Adding/removing devices should generate a new device list (and in the future sign it), while keeping previous ones in database, to have its full history.

The transaction goes as follows:

1. Previous device list timestamp is fetched from database (user table)
2. All user's current devices are fetched from database (device table)
3. A caller-defined function is called to:
   • Mutably modify the "current devices"
   • Return a database update to be performed in transaction
4. A new device list is generated from the modified "current devices" (with new timestamp)
5. The database update is performed in transaction:
   • Current devices are updated (closure-returned DDB operation is performed)
   • New device list is inserted with new timestamp
   • The timestamp in user table is updated to the new device list timestamp
     • Previous timestamp is checked to be equal to the one fetched in step 1. Transaction fails on mismatch

Depends on D10218

Test Plan:
Having this closure:

transact_update_devicelist(db, user_id, |ref mut devices| {
    let new_device = DeviceRow {
      // omitted for clarity
    };
    devices.push(new_device.clone());

    Ok(TransactWriteItem::builder().put(Put::builder()
     .table_name("identity-devices")
     .set_item(Some(new_device.into()))
     .build()
    ))
  }).await
`

Tested a few scenarios, including:

• Normal, the transaction succeeds
• Malfolmed DDB operation - the function returned Error::AwsSdk(err) with my mistake
• Added code that concurrently modifies the timestamp in the users table - transaction fails with DeviceListError::ConcurrentUpdate

Reviewers: michal, varun

Reviewed By: michal, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10219