Add server-id hash to LoginEvent

[RoccoDev]

This adds the serverId hash (that was sent to Mojang during authentication) to a login event for plugins to use.

The reason for this is that while prevent-client-proxy-connections is currently working (previously, a Mojang bug prevented it from working), there is no way to distinguish a proxy block from a regular offline-mode user.

Based on production experience, the proxy block system might prevent legitimate players from joining, especially those in
countries that employ network filtering.

This solution acts as a workaround to allow plugins to manually check for proxies (by sending more hasJoined requests to Mojang), by giving them access to the server ID hash that was generated during authentication.

There are definitely more ways to expose this information, such as:

• Storing the hash in ConnectedPlayer, however this would bloat the Player API with something that's only relevant during login. Additionally, Mojang starts rejecting the same hash after a while.
• Creating a different event, like AuthenticationEvent, dedicated to auth data like the server ID hash.

I think LoginEvent is currently the best compromise to store this data without introducing a new event, let me know what you think.