Provide support for watch multiple namespaces

[ryu-sato]

Hi team,

I would like to enable operator to watch multiple specific namespaces, in addition to the existing capability of watching a single namespace or all namespaces in cluster.

Currently, the operator cannot watch multiple namespaces because it uses the "Namespace" option when instantiating a manager. This option is now deprecated, and you should switch to using the "Cache" option instead, which supports watching multiple namespaces.

Reference: https://github.com/kubernetes-sigs/controller-runtime/releases/tag/v0.15.0

manager.Options.Namespace has been deprecated in favor of manager.Options.Cache.Namespaces.

Therefore, I propose updating the operator to instantiate the manager with the "Cache" option.