Find service offering free SRV records with DNSSEC

[gnarea]

So that we can use it in the public endpoint codelabs.

Unfortunately, freedns.afraid.org doesn't work:

• Their system prevents the creation of records starting with underscores, which is the case with all SRV records. You can contact support to have this restriction lifted, but it takes a few days to get a reply.
• None of its domains (e.g., mooo.com) actually support DNSSEC according to https://dnssec-analyzer.verisignlabs.com/mooo.com