Add support for Endurance mode

[Voyager006]

By Endurance, we mean the game mode where the gap to first place is diminishing for every checkpoint you pass. This is a native game mode in TM2 (which does not technically need to be implemented) but there is no equivalent in United.

Can be confused with CP knockout where players get knocked out for each checkpoint.

CLI will be updated with a new command /ko mode endurance.