Add CurrencyBeacon driver

[ragulka]

Summary

This PR adds a driver for the CurrencyBeacon API, which allows 5000 free requests every month.

Details

CurrencyBeacon supports crypto exchange rates in addition to traditional fiat currencies. However, since this package is geared towards fiat currencies, I am filtering the list supported currencies to fiat only.

Unfortunately, the API does not seem to support filtering the actual rate endpoints by the currency type, only symbols. This means if rates are fetched without a list of symbols, it'll return both fiat and crypto exchange rates.

If this is an issue, it would be recommended to get the list of fiat currencies first and use that to fetch the rates, ie:

$symbols = app(ExchangeRate::class)->currencies();
$rates  = app(ExchangeRate::class)->exchangeRate('EUR', $symbols);

PS. I fixed a tiny issue in one of the existing tests by using the correct URL for a given driver. It doesn't actually affect the test results, but it might cause confusion otherwise.

[ash-jc-allen]

Hey @ragulka! This is awesome! Huge thank you for adding this, I really appreciate the addition. I've just merged it in, so I'll get it released now in v7.2.0 😄