Add native plantimestamp function to Terraform

[liamcervante]

New function will return a constant time representing the UTC timestamp during the plan.

Proposed changelog entry

NEW FEATURES

• Adds a new plantimestamp function that returns the timestamp at plan time. This is similar to the timestamp function which returns the timestamp at apply time.

[alisdair]

Looks good to me. I don't understand the docs linter/build error, though. I imagine we'll have to figure that out before merge.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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