Move Event Model to Run2 namespace for Lamarr use

To use Lamarr with both Run12 and Run3 event models, must be able to compile and talk to both. This means the porting of

Kernel/LHCbKernel

Event/TrackEvent

Event/DigiEvent

Event/RecEvent

Event/PhysEvent

from LHCb run2-patches to Run2Support into the Run2 namespace.

MRs which do this:

• Run2Support!37 : LHCbKernel and TrackEvent
• Run2Support!38 : DigiEvent
• Run2Support!40 : RecEvent
• Run2Support!41 : PhysEvent
• Run2Support!42 : Top level CMakeLists
• Run2Support!44 : Add Event Packer for Run2 namespace wrapped EDM

Already note that LHCb::ParticleID is not ported as there is no difference between master and current. Should this change, it will also need to be ported.

To ensure that the data written with this can be read by older versions of DaVinci (which do not know about the Run2 namespace), one should write the packed event model to DST, at which point downstream software does not need to know about namespacing (@clemenci 's idea).