This commit add IsIPv6Only function inside `internal/ip` package
and moving `geo` package from `internal/geo` to `internal/ip/geo`.
Although it increases server resource usage, checking hostname to IP is
required every time the prober sends a report so that the `ipv6_only`
record in the database is not up-to-date. Previously, this feature did
not exist.
This commit accept IPv6 nodes submission.
When user submit new public node, the server will check IP addresses
from given hostname. If host IP addresses doesn't have IPv4, it will
be recorded as "IPv6 only" node.
Probers that support IPv6 may add `IPV6_CAPABLE=true` to the `.env`
file.
Please note that this feature still experimental and may not being
merged to the main branch.
