Add features to existing SiLA Server/Client package

To add new SiLA features to an existing package, run sila2-codegen add-features Feature1.sila.xml Feature2.sila.xml ... from the package directory (or specify the package directory with --package-dir). See also sila2-codegen add-features: Add SiLA features to existing package.

This will add and update the required files in the generated directory, add ..._impl.py files in the feature_implementations directory, and add the feature to the SiLA Server class in server.py.