The ACLs.Read hook for a resource only allows for the identity of a
resource to be passed in for use in authz consideration. For some
resources we wish to allow for the current stored value to dictate how
to enforce the ACLs (such as reading a list of applicable services from
the payload and allowing service:read on any of them to control reading the enclosing resource).
This change update the interface to usually accept a *pbresource.ID,
but if the hook decides it needs more data it returns a sentinel error
and the resource service knows to defer the authz check until after
fetching the data from storage.
R.B. Boyer