core: add method to enqueue multiple jobs in a single call (#42182)

core: add method to enqueue multiple jobs in a single call (#42182)

Currently only a single job for a single unit can be enqueued
atomically,
so there is no guarantee that, e.g., starting a unit and its socket
at the same time will happen in the same transaction. That forces
callers to 'know' the right order in which to start new units being
installed, or failures will occur. It also means some ordering
constraints are ignored, in case the separate calls are done
in the wrong manual order.

Add a new EnqueueUnitJobMany() D-Bus method that takes a list of units
to start.