feat: optionally apply plan file

[KyleTryon]

Fixes

• apply does not seem to read in plan file #63

To use, the user must:

1. Enable attach-workspace on the plan and apply jobs.
2. The plan parameter must be set on the apply job
   • The plan job now parameterizes the name and path of the plan but defaults to plan.out as before.
   • By leaving the plan parameter on the apply job blank, there are no breaking changes.

[KyleTryon]

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[orb-publisher]

Your development orb has been published. It will expire in 30 days.
You can preview what this will look like on the CircleCI Orb Registry at the following link:
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