FromGenericEnvelope attempts to type-cast a GenericEnvelope instance into a strongly-typed Command Envelope.

ToEnvelope is a convenience function that wraps the provided Command type into an Envelope, with no metadata attached to it.

Handler is the interface that defines a Command Handler, a component that receives a specific kind of Command and executes the business logic related to that particular Command.

Command is a specific kind of Message that represents an intent.

HandlerFunc is a functional type that implements the Handler interface.