Add install mode to watch namespaces matching labels

[owngr]

Hello,

As a Kubernetes operator, I would like to have a mode where the operator watches for events in all namespaces matching a label.

This would allow me to trust the developer to put it only when necessary, and I could still forbid the usage of it by having admission controller rules.

From what I saw a part of it could be easily implemented, the greatest challenge would be if we want the operator to clean after itself if a namespace doesn't match the label anymore, not sure if the other modes already do that, though.

[chen-keinan]

@owngr contributions are welcome

[owngr]

Perfect, I'll look into it