lib/core: introduce two new object types for split xattrs

lib/core: introduce two new object types for split xattrs

This adds two new object types for storing xattrs separately from
content objects.

`.file-xattrs` are regular files storing xattrs content, encoded as
GVariant. Each object is keyed by the checksum of its content, allowing
for multiple references.

`.file-xattrs-link` are hardlinks which are associated to file objects.
Each object is keyed by the same checksum of the corresponding file
object. The target of the hardlink is an existing file-xattrs object.
In case of reaching the limit of too many links, this object could be
a plain file too.