optionally submit testdata via pDNS channel

[satta]

This PR introduces a feature configurable via the new test-domain config entry in the pdns section:

# Configuration for passive DNS submission.
pdns:
  enable: true
  submission-url: amqp://guest:guest@localhost:5672/
  submission-exchange: pdns
  # If test-domain is non-empty, add an extra `A` observation for this rrname to
  # all submissions
  test-domain: heartbeat.fever-heartbeat

which will cause an additional observation to be included with dummy data to each submission:

{
  "timestamp_start": "2021-12-07T18:18:00.029197078Z",
  "timestamp_end": "2021-12-07T18:19:00.063460044Z",
  "dns": {
    "heartbeat.fever-heartbeat": {
      "rdata": [
        {
          "answering_host": "0.0.0.0",
          "rrtype": "A",
          "rdata": "0.0.0.0",
          "rcode": "NOERROR",
          "count": 1
        }
      ]
    },
...
  }
}

This can, for example, be used as a known present observation to implement in-band end-to-end monitoring in a production pipeline.

[0mbi]

Please don't forget to update README.md (if necessary).
I think this would fit in the testing section.

And ofc squash the commits.

Besides that, looks fine for me.

[satta]

Done, thanks for taking a look