Add helm values for Istio Operator `ImagePullPolicy`

Currently imagePullPolicy is hardcoded for the Istio Operator deployment - https://repo1.dso.mil/platform-one/big-bang/apps/core/istio-operator/-/blob/main/chart/templates/deployment.yaml#L34

We should change this value to a helm value (i.e. something like {{ .Values.imagePullPolicy }}) with a default of IfNotPresent and then document the default/override in the table here.

AC:

• Istio Operator deployment updated to allow for overriding the pull policy via helm values
• Ways to override the pull policy added/documented in the table here