smbprovider: Add ReadDirectory and GetMetadataEntry

Add ReadDirectory and GetMetadataEntry methods for SmbProviderClient.
This allows read-only navigation of the share, excluding reading files.

Bug:chromium:757625
CQ-DEPEND=CL:794892

Change-Id: I55d4baa6dec95392774cdea3c07c13989b31e782
Reviewed-on: https://chromium-review.googlesource.com/749897
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523259}
6 files changed