Add ATLANTIS_ENABLE_AUTOPLAN_LABEL config option similar to ATLANTIS_DISABLE_AUTOPLAN_LABEL

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

Describe the user story

I don't want to autoplan PRs by default. When users explicitly want it, they can have their PRs autoplanned.

Describe the solution you'd like

There is already a label that users can set per PR to disable autoplan. I would like to have a label to enable autoplanning per PR.

The label could be configured via env variable similar to ATLANTIS_DISABLE_AUTOPLAN_LABEL

ATLANTIS_ENABLE_AUTOPLAN_LABEL=atlantis-autoplan

and via the cli flag similar to --disable-autoplan-label

--enable-autoplan-label=atlantis-autoplan

When the project has autoplan enabled the label has no affect.

Describe the drawbacks of your solution

Confusion with enable and disable flags.

Describe alternatives you've considered

1. Automatically label all PRs with the ATLANTIS_DISABLE_AUTOPLAN_LABEL value.

race condition with atlantis

1. Users must always explicitly comment atlantis plan

well, this is the status quo that I want to improve on