feat: configure global parallel param

[pseudomorph]

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

Describe the user story

It would be useful to be able to globally configure parallel plan/apply.

Describe the solution you'd like
parallel plans/applies should be globally configurable in a manner analogous to automerge.

Describe the drawbacks of your solution
N/A

Describe alternatives you've considered
N/A