Add support for the managedBy field

[mimowo]

What you would like to be added?

The support for the managedBy field which can delegate reconciliation from built-in controller, to a custom one.

The semantics of the field are:

• whenever the value is set, and it does not point to the built-in operator, then skip reconciliation
• the field is immutable
• the field is not defaulted

Why is this needed?

For context, we have in Kueue the effort (see Support Kubeflow Jobs in MultiKueue) to support kubeflow-training (it will include MPIJob), but it will not be complete without the support for managedBy.

The complete support for the users of MultiKueue (multi-cluster Kueue) means:

• simpler installation (just follow the standard installation path), otherwise only installation of MPIJob CRDs is required
• support for mixed setup in one cluster - some MPIJob could be run by MultiKueue and some by the default operator

The efforts to support the field in:

• k8s Job (alpha)
• JobSet (complete)

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[mimowo]

/cc @andreyvelich @tenzen-y

[mimowo]

xref for the similar issue in the training operator: kubeflow/training-operator#2193

[tenzen-y]

As we discussed in kubeflow community meeting, basically, I'm fine with this feature.
But, I would like to know what @alculquicondor thinks since maybe I don't have enough bandwidth to review this feature.

[alculquicondor]

+1 to this feature.

I also can't promise review bandwidth, but it's something that needs to be done for consistency with the other APIs.

[tenzen-y]

@mszadkow Do you have any plans to work on this issue?

[mszadkow]

/assign