stacks: implement PlanTimestamp method on ExpressionScope

[DanielMSchmidt]

This PR adds plantimestamp to the stacks language. It persists the timestamp in the stack plan and it uses / forces the same timestamp across all components.

Fixes #

Target Release

1.10.x

Draft CHANGELOG entry

ENHANCEMENTS

• Support plantimestamp in the stacks language

[apparentlymart]

I see that this is a draft, so I haven't looked over it in full detail yet, but I just wanted to point out something I noticed on my first read that, if you change it in the way I proposed, would affect lots of different parts of this diff and so seemed worth raising now in case it would get harder to change after the remaining work.

Other than this I just want to make the broad note that for plantimestamp in the modules language we remember the timestamp we generated during the plan phase so that we can return exactly the same timestamp in the apply phase. I think we either need to do similarly here (saving the plan timestamp as part of the stack plan) or we'd need to mark the result of the function as ephemeral so that Terraform doesn't expect it to remain constant between the plan and apply phases. (But then it wouldn't really be the "plan timestamp" specifically anymore.)

[liamcervante]

I think one of the tests is not setting the planTimestamp

[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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