Provide value to scope down storegateway as a values.yaml variable

Provide a means to change the egress ip cidr that is allowed for the storegateway so users can (if they choose) to scope down their cidr for allow egress--> https://repo1.dso.mil/big-bang/product/packages/thanos/-/blob/main/chart/templates/bigbang/networkpolicies/allow-storegateway-egress.yaml?ref_type=heads&blame=0#L16