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.

examples/file-sharing/src/main.rs

@@ -81,10 +81,7 @@ use clap::Parser;
 
 use futures::prelude::*;
 use futures::StreamExt;
-use libp2p::{
-    core::{Multiaddr, PeerId},
-    multiaddr::Protocol,
-};
+use libp2p::{core::Multiaddr, multiaddr::Protocol, PeerId};
 use std::error::Error;
 use std::io::Write;
 use std::path::PathBuf;