This PR has an intention of resolving URLs by using multiple endpoints
that identify an IP address of the service. This is handy if one service
is blocked for some reason.

The detection mechanism follows this logic:

1. It tries to access all services in parallel
2. If service respond with some error (like, no route to host for IPv6),
   then we accurately collect those errors and return a merged one
3. In case of the first IP resolved, we immediately return it.

Also, this PR refactors how access and SNI check are performed.

On some servers ifconfig.co is unreachable (e.g. Hetzner, AdGuard DNS
blocklists), causing 'mtg doctor' SNI-DNS check and 'mtg access' link
generation to fail. New config options allow specifying public IPs
manually, with automatic detection as fallback.

Fixes #405

This method is mostly required for testing