Make lightbeam GPU-compatible

[lukemartinlogan]

@AGENTS.md

We should add GPU support for lightbeam.
On GPU, we should be using hshm::priv::vector with Arena allocator instead.
In addition, for ipc and shm transport types, we should use LocalSerialize/Deserialize instead of cereal.
We will need to make various function HSHM_CROSS_FUN support as well.

Let's write a test very similar to the LocalTransfer tests, except using lightbeam shm transport instead.