[REQ] Python FastAPI Server Code Generator Library Mode

Is your feature request related to a problem? Please describe.

Currently the server stub generated by OpenAPI Generator for Python FastAPI generator only provides support for generating complete server where we need to modify the generated code. The Spring or ASP.NET server generators have options such as delegatePattern or isLibrary in the configuration file which allows us to generate only the interfaces which can be implemented through a concrete class, which lets us easily update the OpenAPI schema and generate new interfaces without modifying existing code by hand.

Describe the solution you'd like

An option to generate the models and APIs as a library which can be published to a package repository and can be consumed by the implementing application.

Describe alternatives you've considered

We are currently updating the generated code through a bash script but that feels like a monkey patching rather than a solution itself as the script depends on the structure of the generated file, which too will change when changing the OpenAPI schema. Implementing a robust solution on the generator itself should be the way to go.

cc: @multani @fa0311 @krjakbrjak @tomplus @cbornet

Start by reviewing the Python FastAPI server generator and the analogous delegatePattern or isLibrary options in the Spring and ASP.NET generators mentioned in the issue. Define how models and APIs should be emitted as a consumable library, then verify that regeneration leaves implementing application code separate and that the generator can publish the resulting package.