Added New Policy Group for Spam Filtering Based on Issue 149

[jkaufman-mitre]

🗣 Description

Added Gmail Policy Group 19 for spam filtering and who can bypass the settings.

Fixes #149

💭 Motivation and context

Comment from SNL

🧪 Testing

✅ 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]

Still in need of TTP Mappings and Drift Rules.