Don't expect monkeypatched nacl.signing.SigningKey instances

[DMRobertson]

A nicer way to fix matrix-org/synapse#11714 (imo)

• Define a new type which holds the data (key: nacl.signing.SigningKey, alg: str, version: str). Either inherit from nacl.signing.SigningKey or introduce a new wrapper.
• Change functions that take a signedjson.types.SigningKey to accept this new data type.
• Major version bump.