Store list of tracks for the PVs

So far the PVs don't contain the list of tracks they are made of, addressing this: https://gitlab.cern.ch/lhcb/Rec/-/blob/master/Tr/PatPV/src/TrackBeamLineVertexFinderSoA.cpp#L781 This is complementary to: LHCb#121 (closed) and might imply a new RecVertex class with SOA.

See also LHCb!2257 (closed)

@adudziak @cmarinbe @freiss