[feature] Command: atlantis lock

[hjkatz]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

See: #3879 (comment)

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So the use-case we have had in the past is via mode: on_plan.

1. Make changes to many many AWS roots (we're about to perform an upgrade)
2. auto-plan, atlantis takes the lock on the PR, now nobody can plan (nor apply) while we're upgrading
3. We make manual TF changes locally and apply via terraform apply (not using atlantis apply), here we are relying on the atlantis PR lock as a communication tactic informing other users that they can't touch TF right now because we are doing things out-of-band
4. We finish the TF changes, now we atlantis plan and confirm there are no changes
5. We atlantis apply (no-op)
6. We merge the PR, atlantis drops the lock, now users can plan/apply on their own PRs

To make a similar process with mode: on_apply we need a way to tell atlantis that other PRs are not allowed to apply any of the roots touched in our PR. Normally this would be coordinated via the atlantis lock (on_plan) on our PR, but since we're using on_apply that atlantis lock no longer exists.

We would ideally still want a way to "claim" the projects via an atlantis lock command, or atlantis apply --no-op or something....

Describe the solution you'd like

New Command: atlantis lock to complement atlantis unlock

This would lock the PR even during mode: on_apply for the project roots included in the PR

Describe the drawbacks of your solution

Describe alternatives you've considered