Add a performance test for Summarize MR AI feature
Adds a basic performance test for summarize an MR AI feature - sends a GraphQL requests and checks for status and resulting requestId.
Note: for this test to work we need to enable add_ai_summary_for_new_mr
in target test environments first (gitlab-org/quality/gitlab-environment-toolkit-configs/quality!168).
Part of https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/8115.
Edited by Anastasia McDonald