feat: add display.render_mode to control DataFrame/Series visualization

[shuoweil]

This PR introduces the display.render_mode configuration option, providing a clearer and more flexible way to control how BigFrames objects are visualized in notebooks and other interactive environments.

Key Changes:

• New Configuration Option: Added bpd.options.display.render_mode which supports three modes:
  • html (Default): Standard HTML table rendering.
  • plaintext: Forces plain text output by removing the HTML component from the mimebundle.
  • anywidget: Enables the interactive anywidget-based table component.
• Migration Path: Maintained backward compatibility for display.repr_mode = "anywidget". The rendering logic now checks both the new render_mode and the legacy repr_mode flags.
• Notebook & Test Updates: Updated existing notebooks and system tests to prefer the new render_mode option while verifying that existing behaviors remain intact.
• New Unit Tests: Added tests/unit/display/test_render_mode.py to specifically verify the mimebundle selection logic and priority across different configuration states.

While repr_mode was previously used to toggle the interactive widget, "representation mode" is a broad term. render_mode specifically addresses the format of the output (HTML vs. Text vs. Widget), allowing for better support for text-only environments and a more intuitive API for users.

Also verified at colab notebook: screen/AHNz4o5Mhb9UHrh

Fixes #<479282023> 🦕

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[shuoweil]

Upon checking, the failed tests are unrelated to our change.