Add sandbox rule for container escape via kernel usermode helper overwrite

[ChrisJr404]

This adds a maturity_sandbox rule that detects a container writing to a kernel usermode helper path such as /proc/sys/kernel/core_pattern or /proc/sys/kernel/modprobe_path, which the host kernel later runs as root and is a known container-to-host escape (MITRE T1611, see https://attack.mitre.org/techniques/T1611/). The condition requires a container context plus CAP_SYS_ADMIN so it stays high signal and low noise, since host boot-time sysctl writes and unprivileged reads do not match.

[poiana]

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[poiana]

Welcome @ChrisJr404! It looks like this is your first PR to falcosecurity/rules 🎉