Add Istio's mesh mode

[nak3]

Since Gateway API v0.4.0, Istio supports Mesh Traffic:

https://istio.io/latest/docs/tasks/traffic-management/ingress/gateway-api/#mesh-traffic

this should be included in the test/controller implementation.

[github-actions]

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[dprotaso]

/lifecycle frozen

[dprotaso]

re-opened this since it should probably be closed by a user vs. a bot