feat: introduce libp2p-identity crate

This patch combines the `libp2p_core::identity` and `libp2p_core::peer_id` modules into a new crate: `libp2p-identity`.

Resolves https://github.com/libp2p/rust-libp2p/issues/3349.

Pull-Request: #3350.

protocols/gossipsub/src/peer_score.rs

@@ -24,7 +24,7 @@
 use crate::metrics::{Metrics, Penalty};
 use crate::time_cache::TimeCache;
 use crate::{MessageId, TopicHash};
-use libp2p_core::PeerId;
+use libp2p_identity::PeerId;
 use log::{debug, trace, warn};
 use std::collections::{hash_map, HashMap, HashSet};
 use std::net::IpAddr;