Handle feature flag APIs

For a long time the Python client generator treated feature flag APIs the same as public APIs. We have recently stop generating feature flag APIs, since they cannot be used on release builds.

This task is about coming up with a good strategy to handle these APIs in the client. Would it be best to generate them but warn when they are used? Do we need to document which feature flag needs to be enabled to use each of these endpoints? Should we let the server issue an error instead of issuing a warning from the client?