export: use io.Writer instead of file

This allows use to use STDOUT directly without having to call open
again, also this makes the export API endpoint much more performant
since it no longer needs to copy to a temp file.
I noticed that there was no export API test so I added one.

And lastly opening /dev/stdout will not work on windows.

Fixes #16870

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

pkg/domain/entities/containers.go

@@ -181,7 +181,7 @@ type CommitReport struct {
 }
 
 type ContainerExportOptions struct {
-	Output string
+	Output io.Writer
 }
 
 type CheckpointOptions struct {