example: add back udp-codec example (#3205)

examples/udp-codec.rs

@@ -1,8 +1,3 @@
-fn main() {}
-
-// Disabled while future of UdpFramed is decided on.
-// See https://github.com/tokio-rs/tokio/issues/2830
-/*
 //! This example leverages `BytesCodec` to create a UDP client and server which
 //! speak a custom protocol.
 //!
@@ -83,4 +78,3 @@ async fn pong(socket: &mut UdpFramed<BytesCodec>) -> Result<(), io::Error> {
 
     Ok(())
 }
-*/