Add callback for TURN authentication success

[renandincer]

Description

Creates a new callback for users to be notified by successful TURN authentication. Together with the authHandler function, this can help determine the ratio of successful vs unsuccessful authentication attemps per username. This information then can be used to implement methods to prevent attacks to guess credentials similar to fail2ban.

There's been some discussion around implementing various usage quotas and limits within this package. An auth success callback can be used to implement some of these given that a succesful source IP address can be reliably linked to usernames. Together with maximum allocation time limit and permission timeouts per RFC, allocation refesh requests (requires auth) and configurable channel bind timeouts, fivetuple -> username usage can be reliably tracked outside of this package.