[C++] Provide a (FileSystem, path) pair to locate files across filesystems

[asfimport]

#8101 (comment)

Paths are sufficient to locate files within a known filesystem, but APIs (for example datasets) do not always have a single known filesystem and in such contexts a (fs, path) pair would be useful.

Reporter: Ben Kietzman / @bkietz

Related issues:

• [C++] Provide utility for copying files between filesystems (is superceded by)

_{Note: This issue was originally created as ARROW-9909. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
It seems this was actually done (the FileLocator struct) in ARROW-9868.