Make Powerdns support rrsets with multiple records more smartly

[dsalisbury]

Description

Hey!

I'm not sure if I've overlooked something here but I've written in in #1967 about the problem I saw where external-dns would continually update a zone even when it was already in the correct state.

From my debugging it appears this is down to the way it fetches its Endpoints from the pdns API. I can't see why it's written as it is but I think my change is legit.

I've tweaked the pdns provider so that when it encounters an rrset with multiple records, instead of building one Endpoint per record, it builds a single Endpoint with multiple targets. This stops it from making the same update over and over.

I will add this is my first time writing any Go so please shout at me if I've done something dumb -- even in this small PR!

Fixes #1967

Checklist

• Unit tests updated
• End user documentation updated

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[njuettner]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsalisbury, njuettner

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