Add support for URLs to OpenAPI definitions

[osteel]

Summary

This PR adds support for URLs pointing to OpenAPI definitions.

Explanation

Fixes #46.

Instead of using OpenAPI PSR-7 Message Validator's methods to create SpecObjectInterfaces objects, I create them through the Reader class directly, which supports both local files and URLs.

Checklist

• I have provided a summary and an explanation
• I have reviewed the PR myself and left comments to provide context
• I have covered the changes with tests as appropriate
• I have made sure static analysis and other checks are successful