Add a callback for the last frame of a round end

[Poggicek]

We may want to store round specific data that cannot exist during a new round (e.g. we schedule timers in the future and want to clear them on round end), currently there's no callback for us to use to cleanup after a round, we can only use round_start which is already too late and multiple events like player's spawning get called beforehand while OnRoundEnd does not fire in every case, e.g. mp_restartgame

Having a callback that always gets called either at the complete start of a round before anything happens or at the complete end of a round would fix this