Use FastAPI instead of OpenAPI generator.

[jkrue]

The current version of the OpenStack GPU Exporter was generated from the OpenAPI specification using the OpenAPI Generator. This approach is rather static, so extending the API requires a lot of effort to implement. FastAPI allows a different approach. The interface specification is written directly in Python, and FastAPI generates an OpenAPI specification from it. To make the GPU Exporter more extensible, it seems sensible to switch to FastAPI.