helm: add resources to load-test the Helm chart

[hugoShaka]

This PR adds load-test resources used to load-test the Helm chart v12 changes.

It contains:

• a node-agent Helm chart that spins a large number of ssh nodes
• a tsh-bench-agent Helm chart that runs a few tsh bench sessions agents
• a few guidelines on how to load-test the teleport-cluster Helm chart

I did not find any other place to put these; the agent Helm charts are not specific to the teleport-cluster Helm chart. They can be reused to set up load-test environments like in the test plan.

I also drafted a couple of Grafana dashboards, but they are not reusable enough to commit today.

[rosstimothy]

This will open a session execute sleep 10 and then leave the session open indefinitely. I'm not sure that is an accurate load test of N sessions. In the real world those sessions are going to be sending data back and forth in response to user/machine input. Something like while ls; do sleep 5:; done doesn't quite mimic a real user session, it does result in input and output being transferred around for the duration of the sessions.

[hugoShaka]

The suggested command makes more sense. We might also want to update the testplan.

teleport/.github/ISSUE_TEMPLATE/testplan.md

Lines 1056 to 1059 in 1637934

	```shell
	tsh bench sessions --max=5000 user ls
	tsh bench sessions --max=5000 --web user ls
	```

[nklaassen]

This seems really handy but there a few parts of the README I would get temporarily stuck on or have to look up to remind myself, left comments on those