Share common code between MuonStationBuilder and MuonStationBuilderCond

This follows MR!59277

Same idea

• We create an Impl clas.
• We move all the common things to it .
• Note that is NOT new code but copy paste of the "common" code
• So it has the same "issues", but now in one place not two (so potentially easer to solve in the future?)
• The 2 derived ones differ on how one "gets" the EventContext
• So we end up with a single 1500 lines impl, rather than 2 ...

This follows some discussion with @jchapman.

PS: I will need to migrate in a similar mannes another piece of code before...

But prb we will need to change the interface of Trk::IDetachedTrackingVolumeBuilder to Trk::IDetachedTrackingVolumeBuilderCond i.e to return unique_ptr. As here it leads to having to "release" things so as to conform to the existing interface.

This is prb 2 MRs down the road but will be needed before "the final step".