Enabling option for watching multiple namespaces

[enxebre]

What would you like to be added (User Story)?

As a cluster service provider managing a multi-tenant environment I would like to setup my capi controllers to watch multiple targeted namespaces

Detailed Description

In a multi-tenant environment where multiple capi controllers are running I'd like to instruct some of them to target only well known namespaces
This should be possible by just making this slice https://github.com/kubernetes-sigs/cluster-api/blob/main/main.go#L162-L163

Anything else you would like to add?

No response

Label(s) to be applied

/kind feature
One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels.

[k8s-ci-robot]

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