Add QR code output to quick tunnels

[elalemanyo]

Summary

When running cloudflared tunnel --url, the quick-tunnel URL is now accompanied by a terminal-rendered QR code. This makes it easier to open the tunnel on a mobile device without having to type or copy the *.trycloudflare.com hostname.

Changes

• Renders the quick-tunnel URL as a half-block QR code in the terminal using ▀/▄/█.
• Keeps the existing URL message unchanged.
• Fails safely: if QR generation fails for any reason, a warning is logged and the tunnel still starts.
• Adds unit tests for URL normalization, quiet-zone padding, error handling, and render correctness.

Notes

• The QR code uses the minimum spec-required 4-module quiet zone.
• Output is filtered to omit all-white quiet-zone rows so the terminal output stays compact.