Packer for PrimaryVertices

We need a packer for the PrimaryVertices monstrosity:

Part of it is SOA container, so it would be good to reuse https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/EventBase/include/Event/SOAPacker.h for that. Will need some help (@ahennequ) to see how we could efficiently do that.

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