core: create abstraction/more properties for the "Exec" part of Unit.StartTransient...

This is a bit of an RFC (but I hope I got it mostly right), @daandemeyer
suggested in
https://github.com/systemd/systemd/pull/42161#pullrequestreview-4336323314
to improve the abstractions around the Exec= in
io.systemd.Unit.StartTransient as we will add a bunch more of those. So
this PR adds first a better abstraction and then uses it. See the
individual commits for details.