codec: add FramedWrite::with_capacity (#7493)

tokio-util/src/codec/framed_write.rs

@@ -47,6 +47,21 @@ where
             },
         }
     }
+
+    /// Creates a new `FramedWrite` with the given `encoder` and a buffer of `capacity`
+    /// initial size.
+    pub fn with_capacity(inner: T, encoder: E, capacity: usize) -> FramedWrite<T, E> {
+        FramedWrite {
+            inner: FramedImpl {
+                inner,
+                codec: encoder,
+                state: WriteFrame {
+                    buffer: BytesMut::with_capacity(capacity),
+                    backpressure_boundary: capacity,
+                },
+            },
+        }
+    }
 }
 
 impl<T, E> FramedWrite<T, E> {