feat(ext): support non-canonical HTTP/1 reason phrases (#2792)

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.

src/ext.rs

@@ -11,6 +11,11 @@ use std::collections::HashMap;
 #[cfg(feature = "http2")]
 use std::fmt;
 
+#[cfg(any(feature = "http1", feature = "ffi"))]
+mod h1_reason_phrase;
+#[cfg(any(feature = "http1", feature = "ffi"))]
+pub use h1_reason_phrase::ReasonPhrase;
+
 #[cfg(feature = "http2")]
 /// Represents the `:protocol` pseudo-header used by
 /// the [Extended CONNECT Protocol].