Get rid of libp2p dependency in sc-network-sync

[dmitry-markin]

libp2p is currently pulled in as a dependency of sc-network-sync due to the use of libp2p::request_response::OutboundFailure in substrate/client/network/sync/src/engine.rs. This should be refactored/replaced by a network backend agnostic type.

[cenwadike]

First time attempting to contribute to polkadot SDK, I'd like to give this a shot

[cenwadike]

Hello @dmitry-markin, I'd like to know what defines an agnostic type.