Migrate developments in MVAUtils to support BDT other the ones from TMVA from !27921 to master
This is migrating the developments from !27921 (merged) to support things beyond TMVA. The rationale for this is in that MR.
The only changes wrt to what requested for 21.2 have to do with keep avoiding the TMVA dependency for the created MVAUtils library in master. More or less repeating some steps of !27691 (merged) since the 21.2 MR changed thigs a bit.
This means things that relate to the TMVAtoMVAUtils.h which was modified to model the new TMVA ctors introduced in the 21.2 MR. These ctors are removed again and hidden again inside TMVAtoMVAUtils.h in kind of factory style, so as to keep avoiding the TMVA dependency in the produced library .so.
The rest should be exactly the same as in 21.2