identify: Sufix litep2p to the agent version for visibility

[lexnv]

This PR adds the (litep2p) suffix to the agent version (user agent) of the identify protocol.

The change is needed to gain visibility into network backends and determine exactly the number of validators that are running litep2p.
Using tools like subp2p-explorer, we can determine if the litep2p validators were responsible for finality lags in kusama.

This reflects on the identify protocol:

info=Identify {
  protocol_version: Some("/substrate/1.0"),
  agent_version: Some("polkadot-parachain/v1.17.0-967989c5d94 (kusama-node-name-01) (litep2p)")
  ...
}

cc @paritytech/networking

[dmitry-markin]

IMO this should go to substrate, not litep2p, to let developers using litep2p set the user agent they want.

[lexnv]

Closing this, will move to substrate, thanks for the review 🙏