A field with each type of withdrawal supported for that asset as a key.
Each type can specify a fields object explaining what fields
are needed and what they do. Anchors are encouraged to use SEP-9
financial account fields, but can also define custom fields if necessary.
If a fields object is not specified, the wallet should assume that no
extra fields are needed for that type of withdrawal. In the case that
the Anchor requires additional fields for a withdrawal, it should set the
transaction status to pending_customer_info_update. The wallet can query
the /transaction endpoint to get the fields needed to complete the
transaction in required_customer_info_updates and then use SEP-12 to
collect the information from the user.