Nightly automated load tests of dev.renku.ch

[olevski]

Run k6 load testing against the renku dev deployment nightly and record the results in Grafana.

The tests will most likely have to run in k8s not in an action because github actions may not have enough resources to launch so many sessions.

There are several existing load tests that can be used for this purpose. We should use and adapt them. In addition k6 has the ability to create very specific metrics (i.e. for specific endpoints) that then can be saved in a time series database that then can be accessed from Grafana.

The load tests can be scheduled nightly with a cronjob in our clusters.