✨ Add WithWaitForHandlerSync to delay reconciliation until handlers sync

[GonzaloLuminary]

Adds WithWaitForHandlerSync option to source.KindWithOptions to ensure reconciliation does not start until all handlers have synced. This is important for controllers that need a global picture of existing resources before making reconciliation decisions.

Examples include the k8s ResourceQuota controller and cluster autoscaler, which do this internally with client-go. This PR brings the same capability to controller-runtime.

Related: Projects like Kueue could also benefit from this feature, though they would need additional API to coordinate handler sync across multiple managers before allowing reconciliation.

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[sbueringer]

I thought this already works out of the box as every Get/List call of the cached reader check if the corresponding informer has synced

[GonzaloLuminary]

I thought this already works out of the box as every Get/List call of the cached reader check if the corresponding informer has synced

We thought the same, but we were not able to get the initial reconciles to happen after all the elements coming from the first List passed through the event handlers. The test added in this PR does not pass in master which indicates that reconciles are happening before all initial elements go through the event handlers. We think that controller-runtime is only waiting for the first List to end before starting the reconcile workers. We'd prefer in some cases for the reconciles to happen after all initial elements have passed through the event handlers.