Summary:
`IOBufQueue` has packing logic, that would copy data instead of chaining `IOBufs`. However, that API unnecessary requires exclusive ownership of the chain.

This diff adds an API that takes reference to IOBuf, copies as much as necessary and then clones/chains the rest.

Reviewed By: yfeldblum

Differential Revision: D14715581

fbshipit-source-id: e047d1c69ca19eec95a7adbca6df740440d23db3