Expose DHT content providers API from sc-network

[dmitry-markin]

Expose the Kademlia content providers API for the use by sc-network client code:

1. Extend the NetworkDHTProvider trait with functions to start/stop providing content and query the DHT for the list of content providers for a given key.
2. Extend the DhtEvent enum with events reporting the found providers or query failures.
3. Implement the above for libp2p & litep2p network backends.

[dmitry-markin]

/cmd prdoc --audience node_dev

[dmitry-markin]

It looks like the CI always gets stuck after a /cmd prdoc command. Another PR where I had to manually kick the CI after /cmd prdoc is #6703.

CC @alvicsam

[lexnv]

LGTM!