Implement bitcoin donations via BTCPay

app/services/btcpay_manager/fetch_exchange_rate.rb

@@ -0,0 +1,14 @@
+module BtcpayManager
+  class FetchExchangeRate < BtcpayManagerService
+    def initialize(fiat_currency:)
+      @fiat_currency = fiat_currency
+    end
+
+    def call
+      pair_str = "BTC_#{@fiat_currency}"
+      res = get "rates", { currencyPair: pair_str }
+      pair = res.find{|p| p["currencyPair"] == pair_str }
+      rate = pair["rate"].to_f
+    end
+  end
+end