feat: add support for ingress backed istio gateways

[DP19]

Description

This adds support for a new annotation on an Istio Gateway to allow getting a target from an ingress object in place of a LoadBalancer service type.

This is particularly useful for GCP's Anthos Service Mesh as they guide users to setup their gateways using a ClusterIP service type with an Ingress routing all traffic to the Gateway.

I don't see other annotation implementations for Source's and I'm looking for some guidance here on where this annotation should live (or if some other signal should be used in place of the annotation)

Fixes #2911

Checklist

• Unit tests updated
• End user documentation updated

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[mloiseleur]

/retitle "feat: add support for ingress backed istio gateways"

[mloiseleur]

Hello @DP19,

Thanks for this PR. It's missing some tests.
I've read https://github.com/kubernetes-sigs/external-dns/blob/master/docs/annotations/annotations.md but I'm unsure if I can suggest a better name. I would tend to prefer a simple name like external-dns.alpha.kubernetes.io/istio-ingress or even external-dns.alpha.kubernetes.io/ingress

[mloiseleur]

/retitle feat: add support for ingress backed istio gateways

[DP19]

Thanks for taking the time to look at this @mloiseleur! Just made some suggested changes and updated the unit tests

[szuecs]

/ok-to-test

[mloiseleur]

/lgtm

[mloiseleur]

/assign @szuecs

[szuecs]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DP19, szuecs

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[legal90]

@DP19 Thank you for fixing that!

@Raffo , @szuecs Could you please release a new version of external-dns and the helm chart with this change included? 🙏
Thank you!

[Raffo]

Release will happen likely next week.