Draft: SM Code Suggestions performance test PoC

Demo of an SM Code Suggestions performance test PoC. Not to be merged.

The aim is to show that it's possible to automate performance tests of code suggestions on a fresh self-managed instance. The only pre-requisite is a subscription that includes the Code Suggestions add-on.

• Adds a job that deploys an SM instance, an AI gateway (with a fake model), and runs a performance test
• Deploys GitLab and AI Gateway via Docker Compose
  • Omnibus-gitlab (nightly image) as the SM GitLab instance
  • Builds an AI Gateway image from the gateway repo
• Use Apache Bench to run the performance test

Part of https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/4120
See also #422136 (closed)

• Remove ML_CS_EE_ACTIVATION_CODE from CI/CD Settings when this is closed.