Create block trade offer

Create a sub-block offer for an existing block trade. Required signers submit their order details and pricing for markets defined in the parent block trade. **Requirements:** - Account must be listed as required signer with active onboarding - Valid order signatures for all markets - Orders must match parent block trade constraints **Includes:** - Individual order signatures per market - Block trade execution signature with expiration - Proper nonce and signature verification data Returns the created offer with unique ID, parent reference, and order details.