Optional underlying web framework support

[kingledion]

couch-rs currently uses reqwest and hyper for its underlying web framework. In the case of an application that uses the actix stack as its web framework, this adds close to 100 dependencies and offers significant bloat to the executable (and, incidentally, about 2-3 minutes of build time with Github actions runners)

This package should optionally use the reqwest/hyper stack or the actix stack as its underlying framework.