[offload] Pool host and shared allocations (#214754)

Route them through a memory manager, like the device allocations. Also,
move the registration as pinned memory to the plugin site since only the
plugin knows if the corresponding host/shared memory is pinned. Improves
performance of affected allocations by ballpark 1,000x.

GitOrigin-RevId: a79b74db04e77f154c68d5f5f87e1ed674e436bf