codec: add Framed::read_buffer_mut (#2546)

Adds a method to retrieve a mutable reference to the Framed stream's read buffer.
This makes it possible to e.g. externally clear the buffer to prevent the codec from
parsing stale data.

tokio-util/src/codec/framed.rs

@@ -195,6 +195,11 @@ impl<T, U> Framed<T, U> {
         &self.inner.state.read.buffer
     }
 
+    /// Returns a mutable reference to the read buffer.
+    pub fn read_buffer_mut(&mut self) -> &mut BytesMut {
+        &mut self.inner.state.read.buffer
+    }
+
     /// Consumes the `Framed`, returning its underlying I/O stream.
     ///
     /// Note that care should be taken to not tamper with the underlying stream