Prepares a sponsored (gasless) transaction with fee payments.

This endpoint builds a transaction where the relayer will pay the network fees on behalf of the user. The user pays fees in a token of their choice (e.g., USDC) instead of the native network currency.

The endpoint accepts either a pre-built transaction XDR or a set of operations to build a transaction from. It returns a prepared transaction that includes:

• The transaction XDR (base64 encoded) ready for signing
• The fee amount in both the fee token and native currency (stroops for Stellar)
• The fee token identifier
• The transaction validity timestamp

After receiving the prepared transaction, the user must sign it and submit it through the standard transaction submission endpoint. For Stellar, the transaction will be wrapped in a fee-bump transaction where the relayer pays the network fees.