Allow user to specify streamed payment currency

[sharon-wang]

An Akita user should be able to specify their desired currency (and toggle between currencies) when viewing their data in the extension. Currently, we only display streamed XRP payments and provide an estimation in USD for simplicity. However, these currencies may not hold meaning to users, depending on their locale. As such, we should allow the user to select which currency they would like to view their data in. We should be able to query an API that has the latest exchange rate to convert to the currency they're looking for.