Add overload to RequestBackup to specify vehicle and peds

[pnwparksfan]

It would be nice to be able to specify a specific vehicle and set of peds to occupy it in the RequestBackup method. An overload, RequestBackup(Vector3, EBackupResponseType, EBackupUnitType, Vehicle, IEnumerable<Ped>), could achieve this. This would allow plugins to take advantage of the built-in backup response system, while also allowing them to send a particular unit as required for a callout or other plugin. For example, one might wish to dispatch a park ranger or army vehicle as backup in a particular area. Being able to still specify the EBackupUnitType is necessary I believe, as that affects things like the ped's props (e.g. SWAT units have vests, others get holsters) and possibly how the AI behave.