Make fastapi dependency optional

[dariogoetz]

Describe why it is important and where it will be useful

The fastapi dependency is only necessary when implementing an "app", i.e. using NextCloudApp or AsyncNextCloudApp. If one is only interested in using the nextcloud API, i.e. NextCloud and AsyncNextCloud, the fastapi dependency is simply "dead weight" (in my specific case, it generated a version conflict because I need to use an older version of fastapi for some reason).

Describe your proposed solution

Is it possible to make fastapi an optional dependency for the app option? This would, however, probably entail moving the NextCloudApp and AsyncNextCloudApp into the ex_app package (which is a breaking change...?).

It would probably also mean that the whole ex_app package could be imported "optionally" (guarded with a try...except block similarly to the uvicorn import in the ex_app.uvicorn_fastapi module).

Describe alternatives you've considered, if relevant

No response

Additional context

No response