enable the use off an external control plane

[rpahli]

What type of PR is this?

/kind feature

This PR enables the use off an external control plane (e.g. https://github.com/clastix/cluster-api-control-plane-provider-kamaji)

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

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[prometherion]

Instead of using a pointer, what about moving the additional field you introduced (ControlPlaneEnabled) here?

With such approach we could introduce the feature smoothly with no breaking changes, and maintain the current state of the pointers.

[rpahli]

The "problem" is that der is a mutation inside the webhook and if this is no pointer then some fields will be filled which makes problems later. To me the question is omitempty means it can be null so the pointer also makes sense to me.

[rpahli]

@marosset @mboersma any updates on this? Can I get a first review?