Support atlantis untaint subcommand

[nitrocode]

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

Describe the user story

Support atlantis untaint subcommand

https://developer.hashicorp.com/terraform/cli/commands/untaint

Similar to #527

Describe the solution you'd like

Allow running terraform untaint using atlantis untaint

Describe the drawbacks of your solution

n/a

Describe alternatives you've considered

n/a