Update envoy-tutorial-istio.md

[Pindar]

Correct typ of port in the MeshConfig for extensionProviders

Why the changes in this PR are needed?

The current state doesn’t work. The change reflects the Typ specified by the documentation:

https://istio.io/latest/docs/reference/config/istio.mesh.v1alpha1/#MeshConfig-ExtensionProvider-EnvoyExternalAuthorizationGrpcProvider

What are the changes in this PR?

Change Typ string to int

Notes to assist PR review:

Further comments:

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[ashutosh-narkar]

Thanks for the contribution @Pindar. Have you verified if the tutorial works as expected with this change? If yes, can also please update https://github.com/open-policy-agent/opa-envoy-plugin/blob/main/examples/istio/quick_start.yaml#L9.

[ashutosh-narkar]

@Pindar please let us know once you've verified that the tutorial works as expected with your changes and we can get this in. Thanks.

[Pindar]

Hi @ashutosh-narkar - I can say with certainty that this works with AKS. I was testing a specific scenario: Azure/AKS#3792 (comment)

I now added another passage, because it took me hours to find out that the service cannot be found without a namespace in certain cases. A note in the tutorial will hopefully help others.

[ashutosh-narkar]

@Pindar can you please squash and sign-off your commit and we can get this in. Thanks!

[Pindar]

@ashutosh-narkar done.