Support shared thread pools for zstd

[joshtriplett]

In gyscos/zstd-rs#283 , I implemented support for sharing a thread pool across compression contexts, to speed up multithreaded compression and avoid having multiple contexts compete for a fixed number of CPUs.

I'd love to have support for this in async-compression.

[NobodyXu]

I think we can expose it as another option for zstd encoder/decoder?

Just like CParameter