Comma-separated list of related resources to embed on the transaction under _embedded, so one call
returns the transaction together with its card subscription, card and cardholder instead of follow-up
calls to GET /card-subscriptions?cardId=, GET /cards/{cardId} and GET /cards/{cardId}/cardholder.
cardSubscription embeds the card subscription exactly as GET /card-subscriptions/{subscriptionId}
serves it (without its own _embedded) and requires the card-subscriptions:read scope.card embeds the transaction's card exactly as GET /cards/{cardId} serves it (without its own
_embedded) and requires the cards:read scope.cardholder embeds the card's cardholder exactly as GET /cards/{cardId}/cardholder serves it and
requires the cardholder:read scope.
Each value is checked separately: a request asking to expand a resource its credentials cannot read is
refused with 403.
The embedded subscription is the card's subscription in the transaction's sub-account: the one from
the transaction's network bulk feed when the transaction has a networkBulkFeedId, otherwise the
card's current subscription, otherwise its most recent one.
Embedded resources show their current values, not the values they had when the transaction
happened.
A key is present only when it was requested and the related resource exists, and it is never null.
A transaction with no card (cardId is null, for example a bank-sourced transaction with no linked
card) has no cardSubscription, card or cardholder. A card without cardholder data has no
cardholder. A card that GET /cards/{cardId} no longer returns (it responds 404, for example
because all of its card subscriptions were deleted) is not embedded: both card and cardholder are
absent. _links are unchanged. Without expand, _embedded is absent.
Send one comma-separated value (expand=cardSubscription,card,cardholder). A repeated expand key,
an empty value or an unknown value is refused with 400.
