add fallback request for req-response protocols (#2771)

Previously, it was only possible to retry the same request on a
different protocol name that had the exact same binary payloads.

Introduce a way of trying a different request on a different protocol if
the first one fails with Unsupported protocol.

This helps with adding new req-response versions in polkadot while
preserving compatibility with unupgraded nodes.

The way req-response protocols were bumped previously was that they were
bundled with some other notifications protocol upgrade, like for async
backing (but that is more complicated, especially if the feature does
not require any changes to a notifications protocol). Will be needed for
implementing https://github.com/polkadot-fellows/RFCs/pull/47

TODO:
- [x]  add tests
- [x] add guidance docs in polkadot about req-response protocol
versioning

polkadot/node/network/availability-distribution/src/pov_requester/mod.rs

@@ -139,6 +139,7 @@ mod tests {
 	use futures::{executor, future};
 
 	use parity_scale_codec::Encode;
+	use sc_network::ProtocolName;
 	use sp_core::testing::TaskExecutor;
 
 	use polkadot_node_primitives::BlockData;
@@ -231,7 +232,10 @@ mod tests {
 							Some(Requests::PoVFetchingV1(outgoing)) => {outgoing}
 						);
 						req.pending_response
-							.send(Ok(PoVFetchingResponse::PoV(pov.clone()).encode()))
+							.send(Ok((
+								PoVFetchingResponse::PoV(pov.clone()).encode(),
+								ProtocolName::from(""),
+							)))
 							.unwrap();
 						break
 					},