Support custom UID/GID

[WolfspiritM]

Hi,

Currently there is no way in windows to provide UID/GID.

I've implemented my own Filesystem but it seems on windows I can't send the uid and gid with my os.FileInfo as that doesn't seem to be implemented at all.

https://github.com/pkg/sftp/blob/dcef9e0d4ab634fede2d5e625cadac1bfc91516b/attrs_stubs.go

Is it possible to provide a way to return that informations from virtual filesystems no matter what OS?

Maybe provide a custom Sys() result object and check if the interface returned from Sys() is that type instead of using syscall.Stat_t directly?

[puellanivis]

Unfortunately, https://golang.org/pkg/syscall/#Stat_t does not export any functions that could be used to type assert on.

That said though, in a type switch, we could check for syscall.Stat_t and if that fails check for something that implements interface{ Uid() uint32; Gid() uint32 } and then use those instead.

[puellanivis]

As part of #432 we will be exposing the ability to return a Sys() of the filexfer.Attributes or FileSet type, and that will be understood and used to allow you to mimic UID/GID values.

The request-example.go server will likely even have a demo of how to do it.