buildAndSimulateSoroswapTx does not simulate using backend

[aristidesstaffieri]

What problem does your feature solve? All simulations should go through the backend for non custom networks in order to ensure we can continue to run Freighter on pubnet|testnet without relying on a publicly available instance of Horizon.

What would you like to see? buildAndSimulateSoroswapTx should use the isCustomNetwork(networkDetails) check like other API calls in order to make the appropriate API call when simulating this tx.

What alternatives are there? N/A