Extend Endpoint to have multiple Targets

[linki]

The Endpoint struct is intended to act as an abstraction over plain DNS records (and Ownership records).

We though it makes sense to group (DNSName, IP) pairs by the DNSName, so that Endpoints look like: (DNSName, [IP_1, IP_2, ...]) with a unique DNSName per Endpoint list.

This would make it easy to work on records related to the same domain name, e.g.

• when using NodePort services with multiple nodes as targets
• when doing any kind of weighting or canary stuff in the future

This issue is for the groundwork that changes Endpoint from having a Target string to Targets []string

Turned out to be more involved than though, work in progress: https://github.com/linki/external-dns/tree/feat/multiple-targets2

[jrnt30]

While revisiting this API, it would be good to integrate #197 there as well. I've looked at starting that a few times and get a bit sucked into the size of the refactor around the changes.

[grimmy]

Since pr 493 was merged, I assume this can be closed?

[linki]

Yes, it can :)