create FastAPI application and use it as the main backend entrypoint

[ricardogsilva]

Let's have a FastAPI app be the main backend entrypoint for web.

This application will provide the frontend with

• the collection configuration properties

It will serve also serve as a proxy for:

• the THREDDS WMS server
• the THREDDS NetCDF Subset Service (NCSS)
• the new API used by ARPAV to relay station observations
• other third-party APIs to be integrated in the system
• the django admin