Add SEP-38 (RFQ API) Support

[JakeUrban]

What problem does your feature solve?

Our demo wallet currently does not support using SEP-38 quotes in SEP-6 or SEP-31 transaction flows.

What would you like to see?

When initiating a SEP-6 or SEP-31 transaction, the demo wallet should:

1. Check if SEP-38 is support
2. Check if the Stellar asset being transacted has off-chain asset exchange pairs
3. Open a modal asking if the user wants to exchange an off-chain asset for the on-chain asset, and if so, have the user select the off-chain asset the user would like to provide/receive.
4. Update the modal to display an estimated exchange rate between the two assets and have the user accept it
5. Create a quote with the anchor's POST /quote endpoint
6. Use the quote in the SEP-6 or SEP-31 transaction as defined by each protocol