Put reqwest implementation in a feature

Proposal: put the reqwest implementation into a feature.

Use case: Someone want to use your crate but without reqwest implement, just by using all types and implement his own request.

No breaking change because the reqwest implementation is a default feature. What you thing about ?

I want to use your crate because you did a really great job in it but I my target (wasm) cannot use your reqwest implementation because the target is fully async.