Allow object-aware external dereference component keys

[ayushshrivastv]

Closes #438

Summary

• add an object-aware ExternalLoader.componentKey overload in both OpenAPIKit variants
• keep the URL-only key path as the compatibility fallback so loaders can implement either form
• compute external reference component keys after loading so the decoded value can inform the key
• add regression tests covering the object-aware override path and the default compatibility path

Testing

• swift test --filter JSONReferenceTests
• swift test --filter ExternalDereferencingDocumentTests

[mattpolzin]

Heads up, I had assigned the issue you are closing with this PR to myself to indicate that I was going to explore the issue (and had not decided what I think a solution should look like yet). I may look over this PR eventually and see if it happens to look just right, but there's a good chance I won't merge this branch. Sorry if the issue assignment was not a clear enough indicator that the issue was spoken for.

[ayushshrivastv]

@mattpolzin Thanks for letting me know.

[mattpolzin]

This was similar to the change I ultimately made, but I made the change against v7 and just went with a breaking change that swapped the type of object out for the object itself. I also didn't create any default fallbacks for component keys -- while I think that the strategy used in the unit tests is informative, I don't think it is useful enough to offer as a default.