🐛 Adding "unmanaged" option for the "controller builder"

[AlmogBaku]

Adding "unmanaged" option for the "controller builder".
Fixes #1821 , see this issue for more details.

The implementation is actually quite small (12 LOC + tests) :P

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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[vincepri]

The builders were meant to be used only for controllers that rely on a manager. If you'd like to use an unmanaged controller, why not call the function directly?

[AlmogBaku]

@vincepri
Currently, the controller builder doesn't support building an unmanaged controller.

To achieve this, developers are required to manually configure and wire the controller and write a lot of code (which is actually the idea of the builder)

[AlmogBaku]

PS. it’s a bit tricky - an “Unmanaged controller” is actually wired to the manager, but just not adding the Runnable to the manager:
https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/controller/controller.go#L87

[k8s-ci-robot]

@AlmogBaku: PR needs rebase.

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[joelanford]

Closing this based on the lengthy Slack discussion on this topic: https://kubernetes.slack.com/archives/C02MRBMN00Z/p1646938170423789