Implement flags for parallel backends

[mdbenito]

Something like:

flag: ParallelFlag = parallel_backend.make_flag()
flag.set()
assert flag.is_set() == True

I don't think it's necessary to generalize to arbitrary values with get/set.

This would be used to signal workers to stop.

[dmeibusch]

Workaround: separately set properties after upload of the artifact rather than using the problematic Groovy overloaded withProperty method that can not be handled by newer Groovy runtimes.

Downsides: two calls vs one call to repository.