Plug-in hook for custom quotes on the ASB
By providing a hook for a plug-in to be queried on a quote request, two features could be added to the ASB:
- Users could determine their exchange rate using a custom algorithm instead of using a Kraken-style websocket plus a fixed spread percentage.
- Users could dynamically set their `min_buy_btc` and `max_buy_btc` limits using a custom algorithm, for example based on their wallet balances.

The current websocket + fixed values solution could be extracted into an exchangable plug-in or be kept as a default solution. Starting a plug-in API might also lay the foundations for future costumization features around a robust ASB core.