Add support for multiple watched namespaces

[chris-pinola-rf]

Adds support for a new environment variable, WATCH_NAMESPACES, which allows users to specify a comma-separated list of multiple Kubernetes namespaces that the k6 Operator will watch for its CRDs. The existing WATCH_NAMESPACE environment variable only supports a single namespace.

I added a new e2e test modeled after the existing testrun-watch-namespace scenario to test the new environment variable.

I verified that the new (and existing) tests pass locally on my k3d cluster.

I also manually inspected the virtual cluster while the new test was running and confirmed that the TestRun CRD from the invisible namespace was not scheduled, but the other two were:

(⎈|testrun-watch-namespaces:N/A)
$ kubectl get testrun -A
NAMESPACE       NAME        STAGE      AGE   TESTRUNID
invisible       k6-sample              84s   
some-ns         k6-sample   finished   84s   
some-other-ns   k6-sample   finished   84s   

Fixes #267

[CLAassistant]

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[chris-pinola-rf]

Hi @yorugac - can you please permit the test workflows to run for this PR?