Generate a random secret instead of the hardcoded one

Currently the secret is hardcoded:
https://github.com/chatmail/chatmail-turn/blob/7f7cc0c0f24108dee6acc62e00694512bd3fe399/src/main.rs#L107

The value is from the default coturn config.

The server should instead generate a random secret on the start. It should also be kept between restarts, because even if the server is restarted chatmail clients will cache the credentials and they should be valid until they expire.