This diff modifies the system resolver to use a getaddrinf transport.
Obviously the transport is a fake, but its existence will allow us
to observe DNS events more naturally.
A lookup using the system resolver would be a ANY lookup that will
contain all the resolved IP addresses into the same response.
This change was also part of websteps-illustrated, albeit the way in
which I did it there was less clean than what we have here.
Ref issue: https://github.com/ooni/probe/issues/2096
This diff refactors the DNSTransport model to receive in input a DNSQuery and return in output a DNSResponse.
The design of DNSQuery and DNSResponse takes into account the use case of a transport using getaddrinfo, meaning that we don't need to serialize and deserialize messages when using getaddrinfo.
The current codebase does not use a getaddrinfo transport, but I wrote one such a transport in the Websteps Winter 2021 prototype (https://github.com/bassosimone/websteps-illustrated/).
The design conversation that lead to producing this diff is https://github.com/ooni/probe/issues/2099
This diff has been extracted from c2f7ccab0e
See https://github.com/ooni/probe/issues/2096
While there, export DecodeReply to decode a raw reply without
interpreting the Rcode or parsing the results, which seems a
nice extra feature to have to more flexibly parse DNS replies
in other parts of the codebase.
