moq-transport: Add lengths to control messages

draft-06 change

See: [PR 520](moq-wg/moq-transport#520), [§6](https://www.ietf.org/archive/id/draft-ietf-moq-transport-06.html#section-6), [§1.3](https://www.ietf.org/archive/id/draft-ietf-moq-transport-06.html#section-1.3-5.2.1)

moq-transport/src/message/mod.rs

@@ -86,6 +86,9 @@ macro_rules! message_types {
 		impl Decode for Message {
 			fn decode<R: bytes::Buf>(r: &mut R) -> Result<Self, DecodeError> {
 				let t = u64::decode(r)?;
+				let _len = u64::decode(r)?;
+
+				// TODO: Check the length of the message.
 
 				match t {
 					$($val => {
@@ -102,7 +105,18 @@ macro_rules! message_types {
 				match self {
 					$(Self::$name(ref m) => {
 						self.id().encode(w)?;
-						m.encode(w)
+
+						// Find out the length of the message
+						// by encoding it into a buffer and then encoding the length.
+						// This is a bit wasteful, but it's the only way to know the length.
+						let mut buf = Vec::new();
+						m.encode(&mut buf).unwrap();
+						(buf.len() as u64).encode(w)?;
+
+						// At least don't encode the message twice.
+						// Instead, write the buffer directly to the writer.
+						w.put_slice(&buf);
+						Ok(())
 					},)*
 				}
 			}