Suggestion: Add `Any` bound to `Connection` type and allow downcasting

Different users of `http-types` will expect an upgraded `Connection` to be able to do different things (some might want it to be `Clone`, some might want to split it, etc.).

A simple way to add some extensibility would be to allow clients to attempt to downcast the connection back to its concrete type. Satisfying the `Any` bound is trivial, but it is nonetheless a breaking change, so would be good to do for the next major version.