WiFi dialog: add script for printing marker section contents

[jotaen4tinypilot]

Related https://github.com/tiny-pilot/tinypilot/issues/131, part (b). Stacked on https://github.com/tiny-pilot/tinypilot/pull/1804.

I realized that we need to add another privileged script for reading files with marker sections, because we are unable to read those directly via the Python process in case those files are owned by the root user.

Usage is demonstrated in the subsequent backend PR; it’s basically …

/opt/tinypilot-privileged/scripts/print-marker-sections /path/to/file.txt

… where the entire invocation has to be unblocked in the sudoers file.