feature: support custom argo exit hooks

[saikonen]

implementing support for running custom functions as exit hooks with argo workflows.

also refactors current lifecycle hook setup to use the same primitives.

Suggested UX for defining custom exit hooks in the flow file:

from time import sleep
from metaflow import step, FlowSpec, Parameter, run_on_finish

def success_print():
    print("Testing some custom exit function.")

def failure_print():
    print("Flow failed.")

@run_on_finish(functions=[success_print], on_success=True)
@run_on_finish(functions=[failure_print], on_error=True)
class NotifyFlow(FlowSpec):
    should_fail = Parameter(name='should_fail', default=False)

    @step
    def start(self):
        print("Starting 👋")
        print("Should fail?", self.should_fail)
        if self.should_fail:
            sleep(60)
            raise Exception("failing as expected")
        self.next(self.end)

    @step
    def end(self):
        print("Done! 🏁")


if __name__ == "__main__":
    NotifyFlow()

[savingoyal]

@exit_hook