Split a payment across several recipients at once

A payment link can now split one payment across up to 10 recipients instead of exactly one. Splitting across 2 or more recipients isn't available on accounts using the seller-led (direct-to-seller) marketplace model; see Platform-led marketplace → Splitting across several recipients.

• payout_shares (an array) on Create payment link and Update payment link is the field to use for a split going forward — a one-item array for a single recipient, up to 10 for several. Every item must use the same mode (all percentage_to_recipient or all amount_to_recipient), the same recipient can't appear twice, and percentages/amounts must sum to 100 or less / the link's amount or less.
• On update, payout_shares replaces the whole split rather than merging with it.
• The response includes payout_shares (the full array) and payouts_share (a legacy mirror of payout_shares[0]) — existing integrations reading only payouts_share keep working unchanged, but new integrations should read payout_shares.