feat(sdk/flutter): add dataset upload APIs and comprehensive examples
Add comprehensive dataset management to the Flutter SDK including:
- Dataset formats: JSONL, CSV, Parquet, Arrow, HuggingFace, TFRecord, WebDataset, Text, ImageFolder, Custom
- Dataset types: text completion, instruction tuning, chat, Q&A, classification, NER, vision, audio
- Upload methods: uploadDataset, uploadDatasetFromFile, createDatasetFromRecords
- Management APIs: listDatasets, getDataset, deleteDataset
- Dataset preprocessing: splitting, shuffling, deduplication, tokenization
- Complete examples showing all formats and use cases
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