refactor(ED25519/keyloader): Change to load from raw bytes.

[tinymahua]

The ED25519 key pair was previously loaded from the PEM format. In the dependency library cryptography, the EDPrivateKey can be used to export in the PEM format, but there seem to be some issues with the export of the EDPublicKey. Loading from the original byte string of the ED key pair can solve this problem and is also more convenient.

[kislyuk]

Thank you for your interest in http-message-signatures. Could you elaborate on what issue you saw with serializing ed25519 keys in the PEM format? The following seems to work for me:

key = ed25519.Ed25519PrivateKey.generate()
key_bytes = key.public_key().public_bytes(
    encoding=serialization.Encoding.PEM,
    format=serialization.PublicFormat.SubjectPublicKeyInfo
)
serialization.load_pem_public_key(key_bytes)

The classes are also deliberately made available for subclassing. If you need to change the loading behavior, you could subclass http_message_signatures.algorithms.ED25519 and change the load_pem_keys method.