docstr: Add generate-skeleton subparser to the output subparser
docstr_tok: ["Add","generate-skeleton","subparser","to","the","output","subparser"]
code: def setup_subparser(cls, subparser):
      """Add generate-skeleton subparser to the output subparser"""
      outputs = sorted(StreamAlertOutput.get_all_outputs().keys())
  
      # Create the generate-skeleton parser
      generate_skeleton_parser = generate_subparser(
          subparser,
          "generate-skeleton",
          description=cls.description,
          help=cls.description,
          subcommand=True,
      )
  
      # Add the optional ability to pass services
      generate_skeleton_parser.add_argument(
          "--services",
          choices=outputs,
          nargs="+",
          metavar="SERVICE",
          default=outputs,
          help="Pass the services to generate the skeleton for from services: {}".format(
              ", ".join(outputs)
          ),
      )
  
      # Add the optional file flag
      generate_skeleton_parser.add_argument(
          "--file",
          "-f",
          default=OUTPUTS_FILE,
          help="File to write to, relative to the current working directory",
      )
  
imports: import json
  from streamalert.shared.logger import get_logger
  from streamalert.alert_processor.outputs.output_base import StreamAlertOutput, OutputCredentialsProvider
  from streamalert_cli.helpers import user_input, response_is_valid
  from streamalert_cli.outputs.helpers import output_exists
  from streamalert_cli.utils import CLICommand, generate_subparser
label_desc: Use image node_id 1 for calling the OutputGenerateSkeletonSubCommand obj's underlying member method code with example usage: obj.setup_subparser(cls, subparser) without return types