Save the caller's answers to custom questions about a partner

Replaces what the caller company answered about company :id to each question in the body — one item per question, answer for the question itself or options for a choice one — and answers those questions as GET …/c2c-custom-questions lists them. As legacy coerced it, true and numbers are stored as text and false, 0 and '' as no answer. 404 not_found unless every question is one of the caller's own or a default one, not removed; legacy failed only when none was, and still wrote the rest. 400 invalid_option for an option its question does not have: only a NUMBER_DYNAMIC_UOM / NUMBER_DYNAMIC_CURRENCY question has its options replaced by the ones sent, to hold the picked unit — legacy replaced every question's options. 409 custom_identifier_answer_unique_per_company when a question with an apiFieldIdentifier gets a value the caller already gave another partner. Only the caller's own answers are replaced; legacy also deleted other companies' answers to a default question. Legacy answered a bare array; this answers { data, metadata }.