Add Crystal::System::Addrinfo

[HertzDevil]

Moves the platform-specific code into a separate module, so that implementations other than LibC.getaddrinfo can be added without cluttering the same file (e.g. Win32's GetAddrInfoExW from #13619). Depends on #14956.

[ysbaddaden]

What about moving the yielding getaddrinfo(&) as a system method? That would abstract the next/free logic.

[straight-shoota]

thought: Maybe getaddrinfo(&) should actually be pushed down entirely to the implementation level. Both current implementations work by iterating abstract handles. But that doesn't have to be the case for all implementations.
It's fine to leave it as is for now, though. We can adapt when the need arises.

[straight-shoota]

thought: I'm wondering if it really makes sense to split the getaddrinfo implementations for Unix and Windows.
The function is almost identical between POSIX and WinSock. So we have a lot of duplication in the handling code. IMO the merged code was actually quite readable with some target-specific conditionals here and there.

[HertzDevil]

The function is almost identical between POSIX and WinSock

Don't worry, GetAddrInfoExW will look different enough

[straight-shoota]

So the plan is to switch entirely to GetAddrInfoExW in a follow-up? Sounds great, then!

[ysbaddaden]

My intent was for the system API to be the yielding getaddrinfo(&) and skip the next/free methods + passing an object (internal détail). Each system implementation will be free to iterate the resolved addresses as they need.