PSS: Validate the backend or state store configurations described by plan files

[SarahFrench]

This PR is blocked by review of https://github.com/hashicorp/terraform/pull/37946, https://github.com/hashicorp/terraform/pull/37956, and https://github.com/hashicorp/terraform/pull/37957

Follows https://github.com/hashicorp/terraform/pull/37248

This PR adds validation to stop Terraform writing or using a planfile that has incomplete descriptions of the backend or state store configuration.

Target Release

N/A

Rollback Plan

• [x] If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• [ ] This change is user-facing and I added a changelog entry.
• [x] This change is not user-facing.

[github-actions[bot]]

The equivalence tests failed. Please investigate here.