kad: Implement GET_PROVIDERS command

[lexnv]

Implement the GET_PROVIDERS command:

• spec: https://github.com/libp2p/specs/tree/master/kad-dht#content-provider-discovery

This command is similar to GET_VALUE. One possible solution is to extend GetRecordContext to support two different messages:

litep2p/src/protocol/libp2p/kademlia/query/get_record.rs

Lines 83 to 84 in a27d007

	#[derive(Debug)]
	pub struct GetRecordContext {

litep2p/src/protocol/libp2p/kademlia/query/get_record.rs

Lines 117 to 118 in a27d007

	let kad_message = KademliaMessage::get_record(config.target.clone().into_preimage());

[dmitry-markin]

Superseded by #201 & #202.