[QA] Toggle Gitaly N+1 detector via feature flag and run tests

What does this MR do?

Allows the QA framework to enable a feature flag, run the tests, and then disable the flag.

Requires gitlab-qa!227 (merged) to be merged at the same time so that the rspec options are handled correctly.

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10324

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/57911
• Closes gitlab-qa#374 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer