Starts a bulk enrollment kickoff for a whole card program. The request records the
company, the card program admin, an optional bank contact and program size, and the
admin's consent, and Astrada's onboarding team takes it from there with the issuing
bank. The Unified Enrollment SDK calls this endpoint from its kickoff wizard
(bulkFlow: "kickoff"); you can also call it directly.

Nothing is enrolled yet at this point. Once the bank has issued the network
reference, register the feed with POST /network-bulk-feeds and pass this
enrollment's id as networkBulkEnrollmentId; the enrollment then moves to
feed-created and links to the feed.

The subaccount must belong to the calling account. The BIN's bulk eligibility is not
re-checked here. A validation failure or an unknown subaccount answers 400.