Implement Policy API Check for Gmail 18

[adhilto]

Prerequisites

• This issue has an informative and human-readable title.

💡 Summary

GWS.GMAIL.18.1, GWS.GMAIL.18.2, and GWS.GMAIL.18.3 are all currently manual checks. However, after a cursory look it appears like the policy API does have the data we need to check them.

Motivation and context

Currently these controls aren't checked at all by ScubaGoggles

Implementation notes

See https://docs.cloud.google.com/identity/docs/concepts/supported-policy-api-settings. Note the various "SpamOverride" settings.

Acceptance criteria

• Confirm whether or not we have the data we needed for those controls
• Implement policy API based checks for the ones we can.