refactor(http/retry): ForwardCompatibleBody::poll_frame()

in the same manner as the existing `frame()` method mimics
`http_body_util::BodyExt::frame()`, this commit introduces
a new `ForwardCompatibleBody::poll_frame()` method.

this allows us to poll the compatibility layer for a `Frame<T>`.

Signed-off-by: katelyn martin <kate@buoyant.io>

Author: cratelyn

linkerd/http/retry/src/compat.rs

@@ -1,6 +1,11 @@
 //! Compatibility utilities for upgrading to http-body 1.0.
 
 use http_body::{Body, SizeHint};
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
 
 pub(crate) use self::frame::Frame;
 
@@ -54,6 +59,18 @@ impl<B: Body> ForwardCompatibleBody<B> {
     }
 }
 
+impl<B: Body + Unpin> ForwardCompatibleBody<B> {
+    #[allow(unused, reason = "not yet used")]
+    pub(crate) fn poll_frame(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<Frame<B::Data>, B::Error>>> {
+        let mut fut = self.get_mut().frame();
+        let pinned = Pin::new(&mut fut);
+        pinned.poll(cx)
+    }
+}
+
 /// Future that resolves to the next frame from a `Body`.
 ///
 /// NB: This is a vendored stand-in for [`Frame<'a, T>`][frame], and and can be replaced once