Make argo events sensor namespace configurable

[dhpollack]

• Add ARGO_EVENTS_SENSOR_NAMESPACE to config (defaults to KUBERNETES_NAMESPACE )
• Add sensor_namespace parameter to argo client
• Inititalize argo client with sensor namespace parameter when creating sensor

Resolves #1455

[dhpollack]

In our set up at work, we don't have event buses in the namespaces where the workflows run. Thus the "normal" behavior is that if one tries to deploy a workflow (argo-workflows create) with the @trigger or @triggered_on_finish decorators then I receive an error from kubernetes that the eventbus does not exist.

With this PR, I was able to test specifying a namespace and not specifying a namespace. When I did not specify a namespace, I got the same behavior as the master branch (i.e. kubernetes error). When I specified the argo events namespace, the argo workflow and sensor successfully get created. When triggered, the triggered workflow runs in the same workspace as the triggering workflow (not the namespace of the sensor) which is the desired behavior.

[jmmclean]

bump on this please; looks to be approved, just maybe some formalities around getting it merged/deployed

[jmmclean]

Bueller? Anyone looking into this PR - its approved but likely need some maintainers to merge/release it