Stand-alone quote endpoint for cross-asset payouts. The customer
submits the destination beneficiary and either a source-side or
destination-side amount, and receives a fresh quote — rate, fees,
and expiry — to review before deciding to create a payout.

The returned quote_id is then passed to
POST /companies/{company_id}/wallets/{wallet_id}/payouts to
bind the quote to a payout — at that point the quoted source
and destination amounts (and rate) become the committed values.

Mirrors the existing FX-conversion quote pattern
(POST /conversion-quotes → POST /conversions {quote_id}).