Add custom transformers integration

[abbbe]

Summary

• support local transformers pipeline via new custom client
• list custom client in CLI provider list
• allow selecting custom provider in interactive mode
• handle custom provider in fuzzing/interactive flows
• add lightweight flan-t5-small pipeline implementation
• document local provider and update dependencies

Testing

• pip install transformers==4.38.2 loguru==0.7.2 --quiet
• pip install -e . --quiet
• pytest -q

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https://chatgpt.com/codex/tasks/task_e_68485213a50483329cd15fa2d9cadf41