Support JSON values in annotation k8s.v1.cni.cncf.io/networks

Multus allows users to specify the value of the k8s.v1.cni.cncf.io/networks annotation in JSON form. Maistra uses this annotation to enable Istio CNI. If the annotation already exists, the Maistra operator assumes the value is in text form and simply appends the Istio CNI plugin name to it. The operator should also support JSON values, as many customers use the JSON form to specify additional properties for their own CNI plugins.