feat: implement Sysctl Hardening Breaks Network Stack lab (#53)

[HimanM]

Description

Implements the new troubleshooting lab for issue #53 (Sysctl Hardening File Breaks Network Stack). In this lab, a custom sysctl hardening configuration /etc/sysctl.d/99-hardening.conf blocks local pings (net.ipv4.icmp_echo_ignore_all = 1) and contains an obsolete key net.ipv4.tcp_tw_recycle = 1 which causes errors when reloading sysctl. The user must find the invalid/obsolete key, comment or remove it, and set the ping parameter back to 0.

Type of change

• New Lab

Lab Requirements (If submitting a new lab)

• The lab.yaml is fully populated.
• The cloud-init.yaml correctly breaks the environment.
• The verify.sh script correctly asserts if the environment is fixed.
• The solution.md provides the step-by-step solution to fix the environment.
• The question.md provides the question for the lab.
• Follows the LAB_FORMAT.md guidelines.
• MANDATORY: A working solution.sh script is provided to automatically fix the environment. (This is required for our CI verification pipeline).

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

Closes #53