Support specifying a namespace to watch

[lfrancke]

Currently this operator watches resources in all namespaces.
I'd like this to be configurable so I can specify which namespace to watch.

This should be a clap argument (which then can be provided on the command line or in an env var) called --watch-namespace.
It is okay to only take a single namespace for now.

• Implement the above description
• Document the default behavior and the new parameter

See stackabletech/issues#162 for the overarching epic