[REST client support] OpenAPI generator research spike

The OpenAPI generator project is an actively developed, Java-based, Apache 2 licensed project hosted on Github. 

OpenAPI generator is capable of producing both client- and server-side code, depending on the availability of the generators. Relevantly the current generators also include a "cpp-qt-client" generator, which produces Qt specific client code. The code compiles against the current Qt 6 'dev' (only some deprecation warnings).

basysKom also made a nice presentation on it a few years ago. A quick browse through the Qt specific mustache templates gives away the main, template-based, idea.

The goal of this task is to have an in-depth look into the existing cpp-qt-client generator and OpenAPI generators in general and answer key questions:

• The current feature set
• The gaps in current feature set
  • In particular: how to improve Qt Quick support
• Compatibility guarantees and release cadence
  • And in particular the fit with Qt's release cadence
  • Notably the OpenAPI customization mecahnisms may play a role