Add GBPT performance tests for Rapid Diffs

Goal

The "in context" metrics for Rapid Diffs (when FF is default ON) will be provided by the 10k ref architecture test suite: https://gitlab.com/gitlab-org/quality/performance/-/wikis/Benchmarks/SiteSpeed/10k.

To do that, we need to add another test to GitLab Browser Performance Tool (GBPT) that tests the rapid diffs (separate endpoint/toggle).