Generate API specification (OpenAPI.json) via Quart-Schema

[pamelafox]

It would help developers to know how the HTTP API works if we generate an openapi.json using Quart-Schema. It also helps in confirming confomance with the HTTP Chat Protocol: https://github.com/Azure-Samples/ai-chat-app-protocol

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [X] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)