feat: Add display.render_mode option to control DataFrame visualization

[shuoweil]

This PR introduces a new configuration option, bigframes.options.display.render_mode, to explicitly control how DataFrames and Series are visualized in Jupyter environments. This change decouples the visualization logic from the execution strategy defined by repr_mode.

Key Changes:

• New render_mode option: Added with three supported values:
  • "html" (default): Renders a static HTML table, preserving the existing default behavior.
  • "anywidget": Enables the interactive TableWidget with sorting and pagination.
  • "plaintext": Forces a plain text string representation even in rich environments.
• Preserved repr_mode behavior: repr_mode remains focused on execution strategy ("head" vs "deferred"). Decoupling these ensures that "head" execution can be paired with any visualization style.
• Updated Selection Logic: The repr_mimebundle implementation now respects the render_mode flag to determine which formats to include in the Jupyter output.
• Robust Testing: Added unit tests to verify the selection logic and updated existing system tests to handle tuple-based MIME bundle returns.

Users will see no change in default behavior (Immediate execution + static HTML). However, they now have a clean mechanism to opt-in to the interactive Anywidget experience or force plaintext output by setting bigframes.options.display.render_mode.

Fixes #<479282023> 🦕

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