Encounter Logic: Spawn lance

[CWolfs]

Add the ability to use a 'Spawn Lance' trigger that runs a 'Spawn Lance' result on a successful condition.

This would be useful for triggering lance spawns mid-way through a contract (e.g. enemy lances that show up after a successful objective has been completed).

[CWolfs]

This now exists for custom contract types. Plans are to backport this to vanilla contract types at some point in the future.