refactor(event)!: add ProxyUser{,Server}ConnectedEvent

[joshuasing]

Summary Add ProxyUserConnectedEvent which is dispatched once the user has successfully connected to the proxy and has been connected to a server.

Rename ProxyUserSwitchEvent to ProxyUserServerConnectedEvent, as the new naming makes a lot more sense in the context of how the proxy works and when the event is dispatched.

Changes

• Add ProxyUserConnectedEvent
• Refactor ProxyUserSwitchEvent into ProxyUserServerConnectedEvent (#getFrom() -> #getPreviousServer(), #getTo() -> #getServer())
• Improve javadoc in UserConnectEvent

Checklist

• [x] I acknowledge and agree to the terms of the Developer Certificate of Origin.
• [x] All contributed code can be distributed under the terms of the MIT License.
• [x] I have read the contributing guidelines and code of conduct.
• [x] I have checked the "Allow edit from maintainers" option.
• [ ] I have added appropriate unit tests for my changes.

This pull request contains breaking changes.