This change adds Windows functional test execution in CI and updates
backend handling so windows filesystem error/path semantics map correctly
to expected S3 outcomes.
The only meta supported on windows right now is sidecar, so the tests
in windows mode also skip sidecar skips.
Future work is to address the skips and/or more clearly document
the unsupported/incompatible behavior on windows.
The windows support will still remain best effort, but these tests
should at least flag when future changes introduce incompatible
behavior on windows.
Ben McClelland