Add APIs to get xattrs from disk

I'm aiming to do some more work on the Rust side around `fsck`
like functionality, and this is a useful primitive.  There isn't
a great Rust crate for xattrs, and I think it's better to share this
code.

apidoc/ostree-sections.txt

@@ -143,6 +143,8 @@ ostree_checksum_file
 ostree_checksum_file_at
 ostree_checksum_file_async
 ostree_checksum_file_async_finish
+ostree_fs_get_all_xattrs
+ostree_fs_get_all_xattrs_at
 ostree_create_directory_metadata
 ostree_validate_structureof_objtype
 ostree_validate_structureof_csum_v