Add CDSE credential support

[dbekaert]

This PR adds Copernicus Data Space Ecosystem (CDSE) credential support to MultiRTC.

Changes

• Add cdse.py module for CDSE token authentication
• Integrate CDSE credential handling in __main__.py
• Update fetch.py and sentinel1.py to support CDSE data access
• Add cdse dependency to pyproject.toml

[dbekaert]

@forrestfwilliams I was hoping this might be a faster alternative for users in Europe. Seems to vary a lot depending if one can hit the compressed archive (aka same zip sizes as ASF) or the native compressed archive (double zip sizes). The burst only access is here not included. Though if this is something you have pointers for from the CDSE perspective that would be great.