Get list of available currencies and exchange rate from API

[johnsBeharry]

In order to show the list of currencies when entering an amount to receive, we need to get a list of currencies, and currency codes that we are able to create invoices with.

This list should be loaded from the BTC Pay Server API and cached on when the app starts up.

• Find the available currency codes and exchange rate providers
• Handle instances without an exchange rate provider
• Allow cache refresh to be triggered for a specific currency (default currency in app), so correct amount can be shown on any screen that displays price