Add auto exchanger for mainnet

[Varunram]

for testnet, since we have our in house stablecoin, we can automatically exchange testnet XLM for USD. On mainnet however, we'd need to create an order on the DEX and the fulfil that order in order to make the auto exchanger work. Since the DEX is not on testnet, we'd need to test with real money. Should try once we're done with the test on mainnet.