feat: Added support for merging targets in DNSEndpoint using a label

[kuritka]

What does it do ?

This PR introduces a new strategy for merging targets in DNSEndpoint, activated by adding the label merge-strategy: merge-targets to the endpoint.

If this label is set, ExternalDNS will automatically merge targets across different owner-ids into a single resulting endpoint. No special configuration is needed—just add the label to the relevant endpoints.

Example:

If you have two DNSEndpoints (typically from different clusters or owners):

spec:
  endpoints:
    - dnsName: cloud.example.org
      labels:
        merge-strategy: merge-targets
      recordTTL: 30
      recordType: NS
      targets:
        - gslb-ns-eu-cloud.example.org

and

spec:
  endpoints:
    - dnsName: cloud.example.org
      labels:
        merge-strategy: merge-targets
      recordTTL: 30
      recordType: NS
      targets:
        - gslb-ns-us-cloud.example.org

ExternalDNS will merge the targets in DNS as follows:

- gslb-ns-eu-cloud.example.org
- gslb-ns-us-cloud.example.org

Note:

• It is up to the client/controller to ensure that their DNSEndpoints are kept up to date and synchronized properly.
• It doesnt handle deletes or updates

Motivation

Enable merging of DNS targets from multiple owners by adding a single label, making multi-cluster or multi-controller setups simpler and conflict-free.

More

• [x] Yes, this PR title follows Conventional Commits
• [x] Yes, I added unit tests
• [ ] Yes, I updated end user documentation accordingly
  • please can you navigate me, where I can update doc?

[linux-foundation-easycla[bot]]

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[mloiseleur]

Hello @kuritka,

Thanks for this PR, this is quite a challenge for handling multi cluster. Without delete or update, it's maybe worth to consider other paths. We should probably take the time to discuss the design before starting the implementation.

First, this is quite unusual to implement features using labels. Would you please explain why you didn't just add a new field on the CRD ?

Second, wdyt about implementing a multiple ownership feature ? This would allow update and the complete deletion would happen when the last owner is deleting.

[mloiseleur]

I'll put this on hold until there is a rework on the approach. /hold

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