s3:vfs: add SMB_VFS_READDIR_ATTR()

s3:vfs: add SMB_VFS_READDIR_ATTR()

SMB_VFS_READDIR_ATTR is a last minute hook to fetch additional metadata
for a directory entry when we're already marshalling the SMB reply
buffer.

This would be used, when there's a need to repurpose some fields in the
the reply, like it's done with Apple's SMB2 extension "AAPL".

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>