Rewrite the fetcher for the new API

http://dataservices.imf.org/ was shutdown, now it is https://data.imf.org/

• API docs: https://data.imf.org/en/Resource-Pages/IMF-API

Notes

• until 2025-04 WEO was distributed as Excel/CSV files, now it is distributed as a regular SDMX dataset (which is a good news!)