Virtual accounts: `mode: "CRYPTO"` payout shape

What's new — POST /v1/virtual-accounts/{id}/payout accepts a new request shape, mode: "CRYPTO", that replaces the legacy payment_instructions envelope for crypto-funded payouts. The deposit address returned is the VA's permanent inbound address (no expires_at); recipient_id and supporting_documents become optional.

Impact — Optional. The legacy payment_instructions shape continues to work and still requires supporting_documents. New integrations should prefer mode: "CRYPTO".

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// Before (legacy — still supported)
{
  "payment_instructions": { "amount": "100.00", "currency": "USD", ... },
  "recipient_id": "rec_abc",
  "supporting_documents": ["doc_123"]
}

// After (recommended)
{
  "mode": "CRYPTO",
  "amount": "100.00",
  "currency": "USD"
}

Reference — POST /v1/virtual-accounts/{id}/payout