First (quick and dirty) implementation of an optional `"codegen"` argument to the ServiceXDataset constructor, to test the mutliple code generator functionality in the backend.
This is only a first implementation, next steps will include propagating a default codegen argument from the default config, overwritable by the user's `servicex.yaml` config file, and possibly more elegant modular propagation of different code generators based on the requested dataset type by the user.