[REQ] Have a master sample openapi doc with most of the features common to all generators

Is your feature request related to a problem? Please describe.

• It is quite difficult to figure out generation differences between multiple generators due to the decentralized nature of openapi samples for each code generator.

  Currently, if we look at https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/3_0/ , each generator has its own sample.

• Some bugs also affect other generators and thus with a master copy, we can find them more easily

• Some generators have fewer features than others, and once new features are added, contributors have to manually update the openapi document to see it in action.

• petstore.yaml is insufficient

Describe the solution you'd like

Create a master openapi sample encapsulating most of the features available that can be used to generate a copy of code for all generators. All contributors to openAPI generator can then build upon it.

Describe alternatives you've considered

Additional context

There are, however, some drawbacks. CI actions may need to be changed to accommodate this. As this would require all samples to be regenerated if the document is changed.

Start by reviewing the existing samples under modules/openapi-generator/src/test/resources/3_0/ and comparing them with petstore.yaml. Then trace the CI jobs that validate or regenerate samples. Done means a shared feature-rich OpenAPI sample is defined and the CI workflow accommodates its use across generators.