This defines an extension type used in requests for the client that is
used to setup a callback for receipt of informational (1xx) responses.
The type isn't currently public, and is only usable in the C API.
This adds an internal ability to copy the HTTP/1 reason-phrase and place
it in the `http::Extensions` of a response, if it doesn't match the
canonical reason. This could be exposed in the Rust API later, but for
now it is only used by the C API.
