Client: Provide easy way to get typing support from dynamic client

Use case

I have SiLA-enabled instruments in my lab and can control them with the dynamic Python client. To get proper IntelliSense for client code, I request the feature definitions from the servers, store them in a directory, run the code generator, and then program using the generated Client class.

It would be nice

1. to have a function that requests all feature definitions and stores them in a given directory (e.g. sila2.client.extract_features(client: SilaClient, output_dir: str))
2. to be able to only generate a client package (no feature implementations, no CLI, no server class)