refactor exchange rates (#1847)

* simplify and cache exchange rate

note that exir was removed as a provider

* add binance as provider

* log exception

* add test

* add blockchain.com provider

tests/core/views/test_api.py

@@ -84,6 +84,19 @@ async def test_create_invoice(client, inkey_headers_to):
     return invoice
 
 
+@pytest.mark.asyncio
+async def test_create_invoice_fiat_amount(client, inkey_headers_to):
+    data = await get_random_invoice_data()
+    data["unit"] = "EUR"
+    response = await client.post(
+        "/api/v1/payments", json=data, headers=inkey_headers_to
+    )
+    assert response.status_code == 201
+    invoice = response.json()
+    decode = bolt11.decode(invoice["payment_request"])
+    assert decode.amount_msat != data["amount"] * 1000
+
+
 # check POST /api/v1/payments: invoice creation for internal payments only
 @pytest.mark.asyncio
 async def test_create_internal_invoice(client, inkey_headers_to):