kad: Implement ADD_PROVIDER command

[lexnv]

Implement the ADD_PROVIDER command:

• spec: https://github.com/libp2p/specs/tree/master/kad-dht#content-provider-advertisement
• find k (20) closest nodes to the provided key
• send ADD_PROVIDER message to the closest nodes

Can wrap the FindNodeContext for performing the initial query:

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

Lines 56 to 58 in a27d007

	/// Context for `FIND_NODE` queries.
	#[derive(Debug)]
	pub struct FindNodeContext<T: Clone + Into<Vec<u8>>> {

[dmitry-markin]

Superseded by #201 & #202.