Enable event-local memory pool for vertex and selection types

Makes LHCb::Event::v2::RecVertex support the event-local memory pool and provide an alias LHCb::Event::v2::RecVertices that's like vector<RecVertex> (but propagates the pool handle to the contained vertices too).

Propagates allocators through Zipping::transform and adds get_allocator() to LHCb::Pr::Zip.

Targets the branch of !2270 (merged) so the diff is correct. Hopefully we can merge that one imminently; for now this makes the diff appear correctly.

No dependencies beyond !2270 (merged), but it's probably most efficient to test with Rec!1896 (merged) and Phys!650 (merged).

cc: @sstahl