Deprecate data_generation_v2; rename DataGenerationV3 → TabularSynthesizer

[copybara-service]

Deprecate data_generation_v2; rename DataGenerationV3 → TabularSynthesizer

Replace data_generation_v2 with the new TabularSynthesizer as the primary
in-memory entry point:

• Rename DataGenerationV3 class to TabularSynthesizer (backward-compat alias
  kept).
• init.py now exports TabularSynthesizer instead of generate (from v2).
• pydantic_api.py migrated to use TabularSynthesizer. Categorical possible_values
  are now stringified for v3 compatibility (enums → .value, bools/None → str).
• bin/main.py migrated from dpsynth.generate() to dpsynth.TabularSynthesizer.
• BUILD deps updated for init and pydantic_api targets.
• README.md updated to reference TabularSynthesizer as the recommended API.
• Examples and colab notebooks updated to use the v3 class-based API.
• data_generation_v2.generate() now emits a DeprecationWarning.