Debug instance for the gossipsub behaviour (#1673)

* Add debug instances for MessageCache and GossipSub behaviour

* Manual impl of Debug for GossipsubMessage

* Add pretty printing of protocol_id in debug

* Use hex_fmt instead of hex

* Inline StringOrBytes helper struct

Since it is used only once here

* Limit data of gossipsub msg to 20 bytes

Otherwise they might become very large and useless for debugging

protocols/gossipsub/src/behaviour.rs

@@ -51,6 +51,7 @@ use wasm_timer::{Instant, Interval};
 
 mod tests;
 
+#[derive(Debug)]
 /// Network behaviour that handles the gossipsub protocol.
 pub struct Gossipsub {
     /// Configuration providing gossipsub performance parameters.