add kick-when-online configuration option

[lipind]

Is your feature request related to a problem? Please describe.

Multiple proxy players can use two accounts (with the same name) to access the server simultaneously, which may cause some bugs,

I thought it was harmless before

But yesterday, on my server, the player accessed both Aproxy-Cserver and Bproxy-Cserver simultaneously

When a player is kicked out It has reached a state of "invisibility", which I analyze to be related to packet sending errors

Can you add a kick when online option？ I think this is missing from ConnectorPlugin

Describe the solution you'd like

When player tries to join with the same account "from another location" (using another computer or even another game window), he cannot join the server, and the client that is already connected to the server remains not kicked.

kick-when-online = false（I think this is similar to the logic of RedisBungee and more convenient for the public）

Describe alternatives you've considered

There can only be one player (with the same name) on a proxy's server like ABCDserver in A-proxy,EFGHservver in B-proxy. ABCDserver only be one player (with the same name) Newly joined players(with the same name) will be kicked out

Additional context

Online mode players can also exploit this bug