Add support for collection export commands

[jfrancoa]

Add CLI support for the new collection export functionality from the Python client (weaviate/weaviate-python-client#1981).

New Commands

• create export-collection - Create a collection export
• get export-collection - Get export status
• cancel export-collection - Cancel a running export

Supported Options

• --export_id - Export identifier
• --backend - Storage backend (filesystem, s3, gcs, azure)
• --file_format - Export file format (parquet)
• --include / --exclude - Collection filtering
• --wait - Wait for completion
• --json - JSON output

Tasks

• Add defaults dataclass
• Add ExportManager
• Add CLI commands (create, get, cancel)
• Add unit tests
• Add integration tests