Expose DHT content providers API from `sc-network` (#6711)

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.

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Co-authored-by: GitHub Action <[email protected]>
Co-authored-by: Alexandru Vasile <[email protected]>