The owner object embedded in a Stripe Tax ID.
Reachable from LatticeStripe.TaxId.t/0. :type says which of the sibling fields is
populated — a tax ID may belong to an account, an application, or a customer — so
match on :type rather than probing the fields for a non-nil value.
Embedded value struct: fields are additive. Keys Stripe adds later appear under
:extra rather than being dropped, so a new field never breaks decoding.