Backport of Pass ephemeral variables to terraform apply into v1.10

[github-actions]

Backport

This PR is auto-generated from #35903 to be assessed for backporting due to the inclusion of the label 1.10-backport.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@DanielMSchmidt
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "90973698bf66aadcdbfe319d85b838c6e530745a", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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This PR improves how we can pass values to ephemeral variables in the apply command, now CLI flags, environment variables, and vars files are supported. We will deal with interactive inputs in a separate PR to keep this one smaller.

Jira: TF-21848

Target Release

1.10.x

Draft CHANGELOG entry

ENHANCEMENTS

• Allow apply-time ephemeral variables

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[DanielMSchmidt]

The skipped commit was a merge commit

[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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