IPv6 support

[MartinKolarik]

I'm opening this to track the requirements for IPv6 support.

Targeting IPv6-enabled hosts in tests:

• [ ] Being able to specify IPv6 address in all relevant inputs.
  • [ ] This must automatically filter the probes to only those that do support IPv6 (and IPv4 addresses must filter to those that support IPv4)
  • [ ] On the probe side, we need to test the output parsing for each test type.
• [ ] When a domain is specified, there must be a way to choose the connection type: auto/ipv4/ipv6.
  • [ ] TBD: Do we auto-select the probes based on this, or do we add tags visible to users?
• [ ] The anti-abuse system must support IPv6.

Probes running in dual-stack and IPv6-only networks:

• [ ] Probes detect which connection types they support for tests and report this to the API.
• [ ] There is only one connection to the API via either IPv4/IPv6. The selection is up to the probe. This doesn't affect the user's ability to run tests of the other type.
• [ ] Infrastructure: The LB and/or API must accept IPv6 connections.
• [ ] Infrastructure: The probe update system (both docker and code) must accept IPv6 connections.