Allow additional commands to be injected directly on the Runner()

[romain-intel]

This allows extensions to add classes with associated methods like Runner("myflow.py", ...).cmd().subcommand()

As an example:

class RunnerCLI(object):
    name = "environment"

    def __init__(self, runner: "metaflow.Runner"):
        """
        Environment command for the flow. Requires the conda environment.
        """
        self.runner = runner

    def resolve(
        self,
        steps_to_resolve: Optional[List[str]] = None,
        arch: Optional[Union[str, List[str]]] = None,
        alias: Optional[List[str]] = None,
        force: bool = False,
        dry_run: bool = False,
        timeout: int = 1200,
    ) -> Dict[str, Dict[str, ResolvedEnvironment]]:

Would add a "environment" command which takes no argument and add the resolve subcommand.