Add Data at Rest processing policy to Common Controls baseline

[jkaufman-mitre]

🗣 Description

Added Policy 15.2 to Common Controls which is the policy about data processing.

💭 Motivation and context

New setting within Google Admin Console.

Close #429

🧪 Testing

N/A no code changes

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• If applicable, All future TODOs are captured in issues, which are referenced in the PR description.
• The relevant issues PR resolves are linked preferably via closing keywords.
• All relevant type-of-change labels have been added.
• I have read and agree to the CONTRIBUTING.md document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge Checklist

• This PR has been smoke tested to ensure main is in a functional state when this PR is merged.
• Squash all commits into one PR level commit using the Squash and merge button.

✅ Post-merge Checklist

• Delete the branch to clean up.
• Close issues resolved by this PR if the closing keywords did not activate.

[jkaufman-mitre]

Applied suggested changes. Ready for review again? @adhilto

[jkaufman-mitre]

@adhilto What is the status of this PR.