Implement CNB exchange rate provider

[divyambhagchandani]

This implementation fetches exchange rate data from the Czech National Bank's public XML API. The data is parsed and filtered to return rates only for the requested currencies. I chose the CNB's official XML endpoint because it's free, reliable, and maintained by the Czech government. The API updates daily and provides all major currency pairs against CZK.
I used a static HttpClient instance to prevent socket exhaustion through connection reuse. I also added a 10-second timeout since the default 100-second wait is excessive for a simple API call.

Future Improvements
Since CNB rates only update once per day, adding a cache layer would reduce unnecessary API calls. Other potential improvements include async/await for better scalability.