litep2p: Sufix litep2p to the identify agent version for visibility (#7133)

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 validators are
running litep2p nodes.

This reflects on the identify protocol:

```
info=Identify {
  protocol_version: Some("/substrate/1.0"),
  agent_version: Some("polkadot-parachain/v1.17.0-967989c5

 (kusama-node-name-01) (litep2p)")
  ...
}
```

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>