Add StreamWrapper API

[joehoyle]

This allows extensions to register and unregister custom stream wrappers

Still need to add docs.

[danog]

Looks awesome, could you take a look at the clippy failures though? :)

[joehoyle]

Ah this had process-globals as the upstream as it relied on some of that stuff. I think it might be better to revert this merge into process-globals as this is failing quite a few clippy checks. We can then put it in draft

[danog]

I've already fixed everything, no need to revert

[joehoyle]

Ok nice one