WIP: Add artillery load testing script and rake tasks

What does this MR do?

Adds artillery.io script for load testing using data generated by the generate_perf_testdata rake task.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57913

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