More informed external dereference component key function

[mattpolzin]

Add a new function to the ExternalLoader protocol that computes a component key with the additional input of the exact object that is being loaded.

This allows implementations of the protocol to use information from the loaded object to determine the key it is stored under if desired.

Add default implementations for both component key functions so either can be implemented but not both are required and remove the less versatile helper from the protocol in v5 of OpenAPIKit (breaking change).