add logout feature

[Guocork]

Fixes #277

[Guocork]

Hi, @kevinaboos , I’ve implemented user account logout in my current commit, but I’ve run into a problem. After logging out, the async_main_loop thread is still using the expired token and throws an 'Invalid access token' error, which means the recently logged-out user can't log in again.

2024-12-24T07:58:55.514036Z ERROR matrix_sdk_ui::sync_service: Error while processing encryption in sync service: Something wrong happened in sliding sync: the server returned an error: [401 / M_UNKNOWN_TOKEN] Invalid access token passed.
src/sliding_sync.rs:1503:13 - Received a sync service state update: Error
src/sliding_sync.rs:1505:17 - Restarting sync service due to error.
src/sliding_sync.rs:1503:13 - Received a sync service state update: Running

However, if I restart the program using cargo run, the start_matrix_tokio function restarts the tokio server and the user can log in normally after logout. So, I’ve come up with a few potential solutions:

1. Cause tokio server run locally, and when the user logs out, the server stays idle and just restarts, waiting for the user to log in again. （This would be the simplest solution, similar to restarting the program.）
2. Modify the current async_main_loop so that when the user logs out, it blocks certain parts of the thread and goes back to checking the login status.
3. Restart the async_main_loop thread itself.

I haven’t tested solutions 2 and 3 yet, so I’m not sure if they’ll work. What do you think about these ideas?