CG-2139 | expose anon search allowed

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do?

We are now exposing the anonymous_searches_allowed setting and allowing for update. This PR also adds in testing for getting the settings and performing updates.

References

#2139