feat(k8s): sidecar containers

[michaelbeaumont]

This PR adds a .experimental config flag. The feature isn't really experimental but it shouldn't be the default yet since the feaure is still beta.
It adds a new job to the matrix that runs the Kubernetes tests with the features enabled.
Question: Do we think it's ok that it runs only on master or should it be run on PRs?

Note: all the added lines are the golden files for injector tests with sidecars enabled.

Checklist prior to review

• Link to relevant issue as well as docs and UI issues -- Closes add support for k8s sidecar-containers #7541
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
  • Don't forget ci/ labels to run additional/fewer tests
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label) --

[slonka]

CI seems to not be running. I'll do "update branch" to see if that triggers a that.

[slonka]

That helped.