Add init-time policy evaluation

[dsa0x]

This is part of a stacked series to upstream the policy work in smaller, reviewable pieces:

• [Add policy client and protocol plumbing #38514]
• [Add policy result types and diagnostic metadata plumbing #38515]
• [Wire policy evaluation into Terraform core runtime #38516]
• [Add CLI and backend policy wiring for plan apply and query #38518]
• [Add init-time policy evaluation #38519] (this)

This PR adds policy evaluation during terraform init. That includes evaluating policy during module installation and provider installation, wiring the init flow through the policy client, and adding the init-specific view/test support needed to surface policy outcomes correctly in that stage of execution.

Contrary to the plan/apply workflow, policy failures during init would result in a non-zero exit of the terraform command.

Included here

• init-time module policy evaluation
• init-time provider installation policy evaluation
• initwd hook changes needed to support policy checks during module installation
• init command wiring
• init-specific view support for policy results
• init-specific tests

Target Release

1.16.x

Rollback Plan

• 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.
• This change is not user-facing.

[github-actions]

Changelog Warning

Currently this PR would target a v1.16 release. Please add a changelog entry for in the .changes/v1.16 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.

[SarahFrench]

I jumped ahead a bit. but I took a glance at this PR and realised I could give some feedback that might be useful if you're blocked while other PRs are under review.

To be explicit, I'm planning to continue focusing on the earliest PR that's open for review out of the stack, so I may not follow up here again until we reach this PR's review.