feat(http2): Implement Client-side CONNECT support over HTTP/2 (#2523)

Closes #2508

src/body/length.rs

@@ -3,6 +3,17 @@ use std::fmt;
 #[derive(Clone, Copy, PartialEq, Eq)]
 pub(crate) struct DecodedLength(u64);
 
+#[cfg(any(feature = "http1", feature = "http2"))]
+impl From<Option<u64>> for DecodedLength {
+    fn from(len: Option<u64>) -> Self {
+        len.and_then(|len| {
+            // If the length is u64::MAX, oh well, just reported chunked.
+            Self::checked_new(len).ok()
+        })
+        .unwrap_or(DecodedLength::CHUNKED)
+    }
+}
+
 #[cfg(any(feature = "http1", feature = "http2", test))]
 const MAX_LEN: u64 = std::u64::MAX - 2;