Switch CDDIS downloads from FTP-SSL to HTTPS with NASA Earthdata authentication

[aclel]

This PR migrates CDDIS file downloads from FTP-SSL to HTTPS using NASA Earthdata authentication, providing better reliability, security, and performance for downloading GNSS products.

Migration Guide

Users should:

1. Set up NASA Earthdata credentials in ~/.netrc with entry for urs.earthdata.nasa.gov
2. Update code to use url_folder parameter instead of ftp_folder

Changes Made

Modified Functions

• New authentication system: Added get_earthdata_credentials() function to handle NASA Earthdata login via .netrc file or direct credentials
• download_file_from_cddis(): Rewrite to use HTTPS with requests.Session and NASA Earthdata auth
• download_multiple_files_from_cddis(): Updated to use new HTTPS approach with thread pool
• download_product_from_cddis(): Uses new HTTPS approach

API Changes

• Added url_folder parameter (preferred) while keeping ftp_folder as deprecated
• Both functions now accept username/password parameters for NASA Earthdata authentication
• Maintained backward compatibility with existing parameter names

Dependencies

• Added requests library for HTTPS downloads

Breaking Changes

See Migration Guide above for authentication requirements. All changes are backward compatible. The deprecated ftp_folder parameter still works but issues a deprecation warning.