Endpoints pertaining to a loan application's borrower's and
co-borrower's income sources, presented as a single collection. Each
record's owner identifies whose income source it is: it is required
on create, always returned on read, and cannot be changed afterwards.
Interim identifier scheme
idis an opaque string, stable only for the lifetime of the loan
application's current field ordering. A persisted, stable identifier is
planned as future work.