Add support for OAS 3.0 oneOf discriminators

[simba909]

Problem

When adding an OpenAPI Discriminator Object to my response schema, rswag doesn't use it to disambiguate between similar schemas specified inside of a oneOf. This makes it impossible to express multiple schemas that are potentially ambiguous to the validator.

Solution

Implement support for using a Discriminator Object (if available) to disambiguate between schemas to use for response validation.

Important: I'm not a Ruby developer by trade which means that this code likely can be improved by someone who knows what they're doing 😅 Any and all suggestions are welcome! I'd love to learn more about this. I've opted not to update the changelog or add documentation yet since I'd like to get feedback on the implementation first.

This concerns this parts of the OpenAPI Specification:

• Discriminator Object

The changes I made are compatible with:

• OAS2
• OAS3
• OAS3.1

Related Issues

Links to any related issues.

Checklist

• Added tests
• Changelog updated
• Added documentation to README.md
• Added example of using the enhancement into test-app

Steps to Test or Reproduce

I've added test cases to validate that the library now uses the discriminator object.

[simba909]

@romanblanco Unsure if you're the right person to request review from but maybe you can point me in the right direction 🙂