feat(datasets): add 100+ dataset integrations with sample() and export utilities

[karaposu]

Summary

• 100+ dataset integrations across e-commerce, business intelligence, real estate, social media, luxury brands, and more
• sample() method for quick data access without specifying filters
• Filter API with operators: =, !=, >, <, includes, is_not_null, and more
• Export utilities: export_json, export_csv, export_jsonl (or auto-detect with export())
• Dataset metadata discovery via get_metadata()
• Mass test notebook for 87 datasets

Usage

async with BrightDataClient() as client:
    # Quick sample — no filter needed
    snapshot_id = await client.datasets.imdb_movies.sample(records_limit=5)
    data = await client.datasets.imdb_movies.download(snapshot_id)

    # With filter
    snapshot_id = await client.datasets.walmart_products(
        filter={"name": "title", "operator": "includes", "value": "laptop"},
        records_limit=10
    )
    data = await client.datasets.walmart_products.download(snapshot_id)

    # Export
    from brightdata.datasets import export
    export(data, "results.json")