Feat Request: Webhook Support for Endpoint Sources

[sleyva-jumptrading]

What would you like to be added:

Similar to DNS providers, external-dns also maintains a list of sources for endpoints. This proposal is to move DNS sources to a webhook based approach similar to providers. This attempts to solve a similar problem to #4347 just for Endpoint sources as well.

Why is this needed:

The ability to integrate with a generic external API via a webhook would significantly enhance the flexibility and utility of External DNS Sources. This feature would enable the system to interact with various external services more dynamically and would reduce the amount of code needed to be maintained in external-dns for various sources (istio gateways, Virtual Services, etc). It could also potentially increase the adoption and versatility of the tool by accommodating different types of external data sources.

[szuecs]

There was also a problem with gateway-api. They have 2 alpha resources that are not available anymore and it blocks our dependency upgrade @Raffo .

I posted this to gateway api channel:

Somehow we need get better for breaking changes even if it's alpha resource https://github.com/kubernetes-sigs/external-dns/pull/4455 shows:

# sigs.k8s.io/external-dns/source [sigs.k8s.io/external-dns/source.test]
Error: source/gateway_grpcroute_test.go:75:18: undefined: v1alpha2.GRPCRouteSpec
Error: source/gateway_grpcroute_test.go:78:20: undefined: v1alpha2.GRPCRouteStatus

I have no good idea right now how to handle this in good way such that alpha resources can iterate quickly without breaking us. One way could be a webhook for sources, I think.

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