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.