fix gateway values object mutation

General MR

Summary

Fixes issue with gateway values being overridden when multiple gateways contain overrides.

Relevant logs/screenshots

Testing with dev-values.yaml

istioGateway:
  values:
    gateways:
      public:
        upstream:
          service:
            loadBalancerIP: 172.28.0.3
      passthrough:
        upstream:
          service:
            loadBalancerIP: 172.28.0.4

Linked Issue

issue

Upgrade Notices

N/A