text: Title: Allow generate command to specify which generator to use in prisma schema
  Body: ## Problem
  
  My python generator is in the same file I use for my js client generator. Running `prisma py generate` fails when I have third party JS generators that are not installed with npm. This is an issue when I try to generate the python prisma client on a container image that doesn't need to generate the third party generators. Currently, I just comment out the third party generators as a work around.
  
  ## Suggested solution
  
  `prisma generate` accepts a `--generator` argument that `Specifies which generator to use to generate assets.`. By exposing this to `prisma py generate`, I'd be able to specify that I only want to build the python generator and skip any other irrelevant generator.
  
  ## Alternatives
  
  Looks like prisma already has this built in! Just need to be exposed to the python cli script.
  