ARROW-10475: [C++][FlightRPC] handle IPv6 hosts

[lidavidm]

uriparser has a more detailed hostData struct that exposes whether the host is IPv4, IPv6, or something else, but doesn't provide apparent ways to convert back. There's also already tests for the current behavior (IPv6 host gets returned without brackets). If we don't care about those tests, we can simply check if uriparser thinks the host is IPv6, and if so, add the brackets back, without writing an IPv6/IPv4 address formatter. (Though that's technically wrong, I suppose - the host is really the unbracketed address, and the brackets only come into play when formatted as a URI.)

Otherwise, here I check if the host looks like IPv6 inside Flight and add the brackets back.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-10475

[pitrou]

There's also already tests for the current behavior (IPv6 host gets returned without brackets)

Those tests are correct, but we can add a function to encode back the host (UriEncodeHost perhaps).

[lidavidm]

I've added a method on the Uri class now to format the host, though the implementation is essentially the naive one still (if ipv6 add brackets else pass through the host).

[pitrou]

I meant a separate function, not a method. The host doesn't necessarily come from a URI.

[lidavidm]

Ah sorry, I think I got what you mean now - updated.

[pitrou]

+1, thank you @lidavidm