feat(ext): make ReasonPhrase::from_static a const fn

Author: seanmonstar

src/ext/h1_reason_phrase.rs

@@ -42,7 +42,7 @@ impl ReasonPhrase {
     }
 
     /// Converts a static byte slice to a reason phrase.
-    pub fn from_static(reason: &'static [u8]) -> Self {
+    pub const fn from_static(reason: &'static [u8]) -> Self {
         // TODO: this can be made const once MSRV is >= 1.57.0
         if find_invalid_byte(reason).is_some() {
             panic!("invalid byte in static reason phrase");