Forward additional information to webhook providers

[hans-m-song]

What would you like to be added:

A way to forward additional information to webhook providers via annotations.

A potential solution is to add support for a wildcard annotation such as external-dns.alpha.kubernetes.io/webhook-* so that they can be exposed via ProviderSpecific here: https://github.com/kubernetes-sigs/external-dns/blob/1bc685774ed38e8b91a8a4529997bbde2593d052/source/source.go#L189

I'm interested in any thoughts / alternate mechanisms and am open to trying my hand at raising a PR.

Why is this needed:

Given in-tree providers are being removed (#4347), the webhook mechanism will need to be more flexible to support all flags from existing providers and any new ones.

The DNSEndpoint CRD seems like it could be useful but it could be difficult to use with third-party charts that don't allow defining additional resources

(And my personal use-case #4230)

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cc @Raffo, @szuecs, and @mloiseleur

[Raffo]

Assigning this myself. We're working on this plan gradually, we're not going to remove things all at once, this seems to be an important feature to add before removing the alpha providers.

[mloiseleur]

@Raffo @hans-m-song Considering that:

1. external-dns is probably more near v1.0 than when those annotations where introduced,
2. webhook is (now) how a provider should be implemented

Wdyt of naming this annotation external-dns.kubernetes.io/webhook-* (without alpha) ?

[mircea-pavel-anton]

I noticed that the latest release of externalDNS (v0.14.2) was released on May 16. This PR was merged on May 24 so it does not appear to be included in the latest release.

Do we have a timeline for releasing this as well? I am working on a Mikrotik Webhook Provider and I was testing out ingress annotations.