Feature: Support spinning a single metaflow step (Rebased)

[talsperre]

To test spin on a new flow you can do the following:

Simple case:

python <flow_name.py> --environment=conda spin <step_name>

Pass in specific pathspec:

python runtime_dag_flow.py --environment=conda spin --spin-pathspec RuntimeDAGFlow/13/step_c/275232971

Pass in custom artifacts via module:

python runtime_dag_flow.py spin --spin-pathspec RuntimeDAGFlow/13/step_d/275233082 --artifacts-module ./my_artifacts.py

Skip decorators (including the whitelisted ones):

python complex_dag_flow.py --environment=conda spin step_d --skip-decorators

Use with Runner API:

    with Runner('complex_dag_flow.py', environment="conda").spin(
        step_name,
        spin_pathspec="<Some Val>",
        artifacts_module='./artifacts/complex_dag_step_d.py',
    ) as spin:
        print("-" * 50)
        print(f"Running test for step: step_a")
        spin_task = spin.task
        print(f"my_output: {spin_task['my_output']}")
        assert spin_task['my_output'].data == [-1]

See the tests for more examples on hot to use this command.

[mt-ob]

I wonder if same should be done for self._store_caches? which is right now an OrderedDict

[romain-intel]

Some smallish comment with maybe a small concern around the transfer of artifacts.