Add API for bot management

[nywek]

Since my gamemode will be utilizing bots and since the usual bot commands will soon no longer work, I would like to suggest having an API that allows adding/removing bots to/from the game. While we are at it, maybe allow kicking any client since kick is probably another command that will no longer be available.

On the gamemode side, it would also be possible to instead just use entities for bots and update them every server tick, so this feature is not necessary/important. Having an actual Client instance attached to them however is a little bit more convenient (at least for me) since I can treat real players and bots more or less the same.