feat(ffi): add hyper_request_on_informational

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.

src/proto/h1/io.rs

@@ -168,6 +168,8 @@ where
                     preserve_header_case: parse_ctx.preserve_header_case,
                     h09_responses: parse_ctx.h09_responses,
                     #[cfg(feature = "ffi")]
+                    on_informational: parse_ctx.on_informational,
+                    #[cfg(feature = "ffi")]
                     raw_headers: parse_ctx.raw_headers,
                 },
             )? {
@@ -678,6 +680,8 @@ mod tests {
                 preserve_header_case: false,
                 h09_responses: false,
                 #[cfg(feature = "ffi")]
+                on_informational: &mut None,
+                #[cfg(feature = "ffi")]
                 raw_headers: false,
             };
             assert!(buffered