RFE: In the DNS prober, allow setting the EDNS0 extended reply size to some size

[siebenmann]

Right now, the DNS prober doesn't allow you to set an EDNS extended reply size, which implicitly leaves it at the original default 512-byte size. This is not large enough for some modern DNS replies for real-world queries, which can cause spurious DNS check failures (when the DNS server you're querying resolved everything but can't send you all of the records, and you're checking something at the end). This is made worse because you can't currently tell in metrics that the reply was truncated (issue #1258). I think the best option is to make the size configurable with another key; if not set, EDNS extended reply sizes aren't enabled at all and everything continues to work as the current situation (in case anyone is actually counting on this behavior of Blackbox DNS probes).