This PR implements all the dialers and resolvers needed to make pre-checks happen. So this task focuses on the following:
1. Implement the DNS probe: call DNSResolver.Resolve(region)
2. Implement the QUIC probe: call QUICDialer.DialQuic (handshake only, no stream opened) and record the result.
3. Implement the HTTP/2 probe: call TCPDialer.DialEdge (TCP + TLS handshake only, no frames sent) and record the result.
4. Implement the Management API probe: call ManagementDialer.DialContext to api.cloudflare.com:443 and record the result.
5. Export edgeDiscovery as EdgeDiscovery in edgediscovery/allregions/discovery.go so the pre-check can reuse the production DNS path.
This sets up the main components to implement the checker.
We previously always preferred region2 as the first region to connect
to if both the regions cloudflared connects to have the same number of
availabe addresses. This change randomises that choice. The first
connection, conn index: 0, can now either connect to region 1 or region
2.
More importantly, conn 0 and 2 and 1 and 3 need not belong to the same
region.
Miguel da Costa Martins Marcelino
Devin Carr
iBug
Sudarsan Reddy
Areg Harutyunyan
cthuang
Dalton
Igor Postelnik
Adam Chalmers