Performance test ActionCable with Puma

This is related to the discussion we had in https://gitlab.com/gitlab-org/memory-team/team-tasks/-/issues/39#note_334231800

This feature is still experimental but eventually we'd need a way to run performance tests for these real-time features.

During the working group meeting, it was also mentioned that a first step here could be to just run the existing performance tests with the feature enabled to make sure that we are not introducing a performance regression.

This would need some configuration to enable the ActionCable Puma server and also the feature flags.