Add feature flag to disable db-writes on git fetch over https [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This adds the disable_git_http_fetch_writes feature flag. It allows diabling some non-essential writes during git fetches hopefully improving the apdex of some of the git endpoints.

Related to gitlab-com/gl-infra/scalability#883 (moved)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done