feat: add request callback config option

[dennis-tra]

This pull request adds a callback hook into the stream handler and will be called on every request.

[guillaumemichel]

IIUC checking whether a field is set is cheaper than invoking an empty function. Otherwise looks good to me 👍🏻

What will be the use case of this new hook?

[dennis-tra]

The use case is for ants-watch so that we can avoid our own fork. IIRC I'm also using a similar hook in parsec (but need to check).

Do you want me to change the code to do a nil check instead? nvmd, you already did that 👍