build
abstract suspend fun buildFeePayer(sender: AccountAddressInput, payload: TransactionPayload, secondarySigners: List<AccountAddressInput> = emptyList(), feePayer: AccountAddressInput? = null, options: TransactionOptions? = null): AptosResult<UnsignedTransaction.FeePayer>
Builds a sponsored transaction. Omitting feePayer uses the zero-address placeholder so an external sponsor can be selected after construction.