feat: Allow checking that the DNS configuration is correct

[RemiBardon]

• Pod address configuration
  • OpenApi description
  • Smoke tests
• DNS setup routes
  • OpenApi description (get_dns_records)
  • Smoke tests
• Network checks
  • DNS records
    • OpenApi description (check_dns_records)
    • Smoke tests
    • Integration tests (network-config.yaml)
  • Ports reachability
    • OpenApi description (check_ports)
    • Smoke tests
    • Integration tests (network-config.yaml)
  • IP connectivity
    • OpenApi description (check_ip)
    • Smoke tests
    • Integration tests (network-config.yaml)
  • All at the same time
    • OpenApi description (check_network_configuration)
    • Smoke tests
    • Integration tests (network-config.yaml)

Edit: After a discussion regarding Nomad during the review of this PR, we have decided not to write integration tests for those features. There are multiple reasons:

1. Running tests on a distant machine that uses DNS config on a public domain is a great integration test, but it's hard to automate. We wouldn't be able to test specific scenarios easily without touching the DNS configuration by hand.
2. We will soon integrate the Prose Pod API in the Nomad configuration of the already-in-production Prose Pod we have at <prose.org>. This will likely reveal things we hadn't anticipated regarding deployment and will make the process more mature. Once we have a Nomad configuration we can use to spawn Prose Pods easily, it will be far easier to create temporary integration test environments.
3. The DNS resolution logic of this PR comes from code written by @valeriansaliou for another project. It works and has been used in production, therefore it's safe to assume it will work for here too without integration tests.
4. When receiving SSE events, Step CI looks at message IDs and suppose they're unique. To test our "checks" routes, we'd have to rewrite how Step CI matches SSE events since we send multiple times the same messages IDs with updated data. This will take a lot of efforts for something we don't need right now.
5. We will implement those integration tests later, and will create an issue for it.