Parse Nested postman collections

[MaggieKimani1]

Fixes #777,
Fixes #787

This PR:

• Refactors the ParseJsonCollectionFile logic to use a recursive solution that helps parse single-level or multi-level deeply nested item arrays containing the request object from which we extract the request URLs.
• Adds unit tests for both single-level nested item arrays and double-level nested arrays for validation
• Logs a warning message to the console when a URL in the postman collection isn't present in the input OpenApi document instead of terminating the entire process
• Updates the conditional operator to infer OpenApiSpecVersion from the diagnostics object if a user doesn't supply the version parameter