Add optional Pydantic model support

[RogerThomas]

Summary

• add an optional pydantic extra
• expand a lone Pydantic BaseModel parameter into CLI options automatically
• construct and validate the model before invoking the target function
• document the integration and add it to the yeetr vs. typer comparison

Verification

• task check
• uv run pytest
• env UV_CACHE_DIR=/tmp/uv-cache uv build
• verified the wheel advertises the pydantic extra
• uv run yeet app.py -n person --tol 0.25