Support external data source import and export

### Feature Summary
Support integration with external data sources so users can both import data from and export data to cloud storage services such as Google Drive, Box, Dropbox, and other providers.

This would help users move data between Texera datasets and external storage platforms more easily, especially when data is already stored in shared cloud folders or is too large to move manually.

### Proposed Solution or Design
Add a unified external data source integration framework that supports both:
- Importing files from external storage into Texera datasets.
- Exporting Texera dataset files to external storage.

The initial implementation can focus on one provider first, such as Google Drive, while keeping the connection layer extensible for additional providers like Box and Dropbox.

### Affected Area

_No response_

No files, tests, or entry points are identified in the issue, so first map the existing Texera dataset import and export paths and review how external connections are represented. Define the scope for one provider, such as Google Drive, and confirm that both import and export work while leaving a path for Box and Dropbox integrations.

Written by the indexing model from the issue text.