CoreDNS Plugin: Add support for setting the default geo

**What**

We need to be able to specify which GEO any traffic from an unknown GEO should be directed to.

Add the ability to the CoreDNS plugin to select the most appropriate GEO if no Geo is explicitly set for an endpoint. This shoud work in a similar way to AWS where it will use the endpoint marked as "default" if one exists.

Currently the plugin will just select one at random if multiple GEOs exist.

Start by reading the CoreDNS plugin's GEO selection path and how it handles endpoints without an explicit GEO. Define how an endpoint marked as the default is selected for unknown GEO traffic, then verify that selection is deterministic when multiple GEOs exist.

Written by the indexing model from the issue text.