DNSLink fallback to gateway

[John-LittleBearLabs]

Has to be turned-off-able through preferences (it is), because of the security implications.

If the DNS TXT request/path fails for one reason or another, then request IPFS gateways resolve the name for you. There's actually a few reasons why this could be helpful:

1. Your system's DNS setup (e.g. resolv.conf) isn't quite reproducible by Chromium so it gave up on reproducing it before it got to DNSClient. None of the other DNS providers inside Chromium support TXT requests.
2. DNS in general where you are might be botched in some weird way. It pretty much has to be a weird way, because it can still reach the gateways.
3. Coincidentally solves the ENS support ticket - one of the first tickets added by not-me. You can't fetch TXT records for a .eth domain through normal DNS, but many gateways do support such a resolver.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.34%. Comparing base (cf03085) to head (4d2d777).

Additional details and impacted files

@@              Coverage Diff              @@
##           regscheme     #115      +/-   ##
=============================================
+ Coverage      50.31%   50.34%   +0.03%     
=============================================
  Files            477      477              
  Lines          16093    16145      +52     
  Branches        1750     1755       +5     
=============================================
+ Hits            8097     8129      +32     
- Misses          7440     7459      +19     
- Partials         556      557       +1     

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[John-LittleBearLabs]

Ignore chromium_edits/ here. The most important changes are in gateway_request.cc , multi_gateway_requestor.cc , and dnslink_requestor.cc . Some stuff in Client and preferences.cc as well, but borderline trivial.