feat(load-test): Add load test for classroom scenario

[li-il-li]

This PR adds a load test that simulates a starting class:

1. Login
2. Fork an example project
3. Start session
4. Wait for session to start
5. Let session run for n seconds
6. Shutdown session
7. Cleanup/delete forked project

Additionally a custom metric + trend is introduced which measures the servers startup time.

I was running into some problems, where I was getting 401 errors forking the project, even after successfull login. This has not been resolved yet.

[olevski]

@li-il-li can you move this over to the notebooks repo? I think it targets pretty much exclusively renku-notebooks components.

EDIT: Actually nvm. I see that this way you can reuse the login function. If you move this then we would have to copy things over. So it is all good.

[olevski]

Ok lets just get this in because the added tests are definitely useful.

I opened an issue to look a bit more into why some requests need retries: #3240