Add metadata linting in PR checks

[patcon]

Not for checking the whole config file, but just checking format of our metadata key, which is being used to keep track of:

1. the repository the code comes from that's using the subdomain, and
2. the maintainer(s) responsible for whatever lives at the subdomain

Could also use the linter to check for has admin control of the domain. I'm currently storing this in a TXT entry at the root level of each domain, as admin=foo.

https://github.com/23andMe/Yamale
https://github.com/nrkno/yaml-schema-validator-github-action

[patcon]

• check that root domain has an admin=username TXT value
• check that every A and CNAME record has:
  • a repository or url key
  • a maintainer key
• check that TXT entries starting with 301 have an entry for ronny's app (and vice versa)
• ensure all domain config files are *.yaml (NOT *.yml)