A node's network identity keys.
Such identity keys can be randomly generated on every startup,
but using already existing, fixed keys is usually required.
Though libp2p uses other crates (e.g. ed25519_dalek) internally,
such details are not exposed as part of libp2p's public interface
to keep them easily upgradable or replaceable (e.g. to ed25519_zebra).
Consequently, keys of external ed25519 or secp256k1 crates cannot be
directly converted into libp2p network identities.
Instead, loading fixed keys must use the standard, thus more portable
binary representation of the specific key type
(e.g. ed25519 binary format).
All key types have functions to enable conversion to/from their binary representations.