Added method for changing client's team using L4DTeam

[armanossiloko]

In clients.inc, there is a native void ChangeClientTeam(int client, int team); which accepts an integer. To make the dev experience easier (and also to make some of the plugins that were made for me compile without having to figure out which team is what integer), adding a new method that accepts the L4DTeam enum.

Hopefully this would make some dev experience "easier" sometimes.