reduce libp2p dependencies

[mattsse]

the libp2p feature pulls in libp2p entirely:

discv5/Cargo.toml

Line 17 in 44051ef

libp2p = { version = "0.53", features = ["ed25519", "secp256k1"], optional = true }

but only identity and MultiAddr are used

discv5/src/node_info.rs

Lines 7 to 11 in 44051ef

	use libp2p::{
	identity::{KeyType, PublicKey},
	multiaddr::Protocol,
	Multiaddr,
	};

which are available as standalone crates:

multiaddr and libp2p-identity

[AgeManning]

Resolved in #255