Add a new extension type `hyper::ext::ReasonPhrase` gated by either the `ffi` or `http1` Cargo
features. When enabled, store any non-canonical reason phrases in this extension when parsing
responses, and write this reason phrase instead of the canonical reason phrase when emitting
responses.

Reason phrases are a disused corner of the spec that implementations ought to treat as opaque blobs
of bytes. Unfortunately, real-world traffic sometimes does depend on being able to inspect and
manipulate them.

Non-canonical reason phrases are checked for validity at runtime to prevent invalid and dangerous
characters from being emitted when writing responses. An `unsafe` escape hatch is present for hyper
itself to create reason phrases that have been parsed (and therefore implicitly validated) by
httparse.

Co-authored-by: silence <[email protected]>
(cherry picked from commit 5fa113e)

(cherry picked from commit a563404)