[Phase 1] Fix bb_create_posting: complete parameter coverage

**Free posting** missing: `postingtext`, `postingaccount_debit`, `postingaccount_credit`, `vat`, `cost_location_two`

**Receipt posting** missing: multi-posting support (`postingtexts[]`, `vats[]`, `amounts[]`, `creditor`, `debtor`)

**Transaction posting** missing: multi-posting support + `oi_receipts_ids_by_customer`