Add API for extracting the inner payload of RawNetworkMessage

I'd like to take out the `payload` of RawNetworkMessage and
then send it to the actual network message processor, but
finds there is no way to do it. This commit adds such an API
to expose the owned value of inner `payload`.

bitcoin/src/p2p/message.rs

@@ -310,6 +310,11 @@ impl RawNetworkMessage {
         Self { magic, payload, payload_len, checksum }
     }
 
+    /// Consumes the [RawNetworkMessage] instance and returns the inner payload.
+    pub fn into_payload(self) -> NetworkMessage {
+        self.payload
+    }
+
     /// The actual message data
     pub fn payload(&self) -> &NetworkMessage { &self.payload }