reqmgr2 get implementation and test (initial version)

This replaces https://github.com/dmwm/WMCore/pull/4712  - squashing all the comit

@zdenekmaxa. I fixed the unittest for software version. However, aux.json need to be updated with more real data. (could you add that).
I would like to move out some of the functions and code from ReqMgr_t test. (We can discuss later).
Still need to add more unittest but it can be merged for now.

Please review.