[OAS] Generator development environment

Lay out the groundwork for developing OpenAPI Generator - based generator (cpp-qt6-client, name TBD). The target is to have a version-controlled shared development environment for developing and testing the new generator.

This means a minimalistic version of:

• A generator capable of producing code
• A helper script to use the generator (instead of using hard-to-remember command-line invocations)
• Test application / autotest to test that the generated code compiles
• Manual test application for testing the generated code's functionality (eventually an autotest)

The manual testing application requires a server-side to work - a server that responds to requests. Two complementing options:

• petstore.yaml is the standard reference used by OpenAPI project. It's possible to run a localhost server that returns and stores values
• colorshop.yaml. Doesn't exist at the moment, but a specification for interfacing QHttpServer's colorpalette server example. In the long run we want to provide a client example application against that example server [0]

[0] And in the even longer run, we want to be able to run the colorpalette client application against a generated server code (a server-side example in the qtopenapi repo)