instead of label auto-validation, move towards GH comment hook

[cquil11]

Right now, when we want to run a test job from a PR, we use the label-validation.yml workflow which parses the labels on a PR to run smoke test sweeps.

In order to give users more control over tests and to not have 3224 labels (which I think is kinda ugly), just have a hook that listens to a PR comment so that users can run commands on a PR to trigger test sweeps. Something like

/sweep <generate_sweep_configs.py command>

Which literally just takes in the CLI command and passes it to the generate_sweep_configs.py file then runs the workflow.

Starter references:

• https://johnnyreilly.com/slash-command-your-deployment-with-github-actions
• https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/about-slash-commands