🐛 Fix builder engage options to merge instead of replace

[stanfordpeng]

Summary

Fixes #93

This PR fixes the EngageOptions behavior in ApplyToFor() and ApplyToOwns() to merge options field-by-field instead of replacing the entire struct. This makes the behavior consistent with ApplyToWatches(), which already correctly merges options.

Problem

When using the multicluster builder with multiple EngageOptions, the last option would overwrite all previous options instead of merging them:

mcbuilder.ControllerManagedBy(mcMgr).
    For(&MyResource{},
        mcbuilder.WithEngageWithLocalCluster(true),
        mcbuilder.WithEngageWithProviderClusters(false),
    ).
    Complete(reconciler)

Due to the implementation in ApplyToFor() and ApplyToOwns(), only engageWithProviderClusters=false would be applied, while engageWithLocalCluster would remain nil and fall back to the default.

This made it impossible to configure hub-and-spoke patterns where you want to watch resources in the local cluster but access remote clusters via GetCluster().

Solution

Changed ApplyToFor() and ApplyToOwns() to merge options field-by-field, matching the existing pattern in ApplyToWatches():

func (w EngageOptions) ApplyToFor(opts *ForInput) {
    if w.engageWithLocalCluster != nil {
        local := *w.engageWithLocalCluster
        opts.engageWithLocalCluster = &local
    }
    if w.engageWithProviderClusters != nil {
        provider := *w.engageWithProviderClusters
        opts.engageWithProviderClusters = &provider
    }
}

This properly merges each field independently while creating new pointer instances to avoid aliasing issues.

Testing

Verified that the hub-and-spoke pattern now works correctly by testing with the following configuration:

mcbuilder.ControllerManagedBy(mcMgr).
    For(&XCR{},
        mcbuilder.WithEngageWithLocalCluster(true),
        mcbuilder.WithEngageWithProviderClusters(false),
    ).
    Complete(reconciler)

The controller now correctly watches only the local cluster while maintaining access to remote clusters via the manager.

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