Add Code Completions Direct Connection Test

Implements https://gitlab.com/gitlab-org/quality/performance/-/issues/610+

Adds a test to verify the performance of Code Suggestions when a client makes use of a direct connection to the the AI Gateway rather than connecting via the GitLab monolith.

To run:

./bin/run-k6 --environment gdk.json --options 60s_2rps.json --tests tests/experimental/api_v4_code_suggestions_completions.js tests/experimental/direct_connection_code_suggestions_completions.js