Add PrivateConfig field to SecureSourceManagerInstance

[ericpang777]

Change description

Adds the PrivateConfig field to the controller

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[ericpang777]

Branched off #3238

@yuwenma Is it okay to let this through despite the example not working in real gcp due to a limitation of the KCC PrivateCACertificateAuthority? The KCC PrivateCACertificateAuthority doesn't enable the CA for some reason, but if you enable it manually, this example would work, so it's not an issue with the logic of SSM.