feat(install): one-liner installer for the Linuxfabrik package repository

[markuslf]

Provide a shell script that auto-detects the OS family, registers the Linuxfabrik package repository and installs the monitoring-plugins package. Goal: replace the four per-distro copy-paste blocks in INSTALL.md with one command that works on every supported distribution.

Target one-liner:

curl -fsSL https://repo.linuxfabrik.ch/install-monitoring-plugins.sh | sudo bash

Script responsibilities:

• Read /etc/os-release to identify RHEL / SLE / Debian / Ubuntu (plus $VERSION_CODENAME on apt families).
• Import linuxfabrik.key (apt: keyring at /etc/apt/keyrings/linuxfabrik.asc; rpm: rpm --import).
• Register the correct sub-repository (rhel/, sle/, debian/, ubuntu/).
• Install linuxfabrik-monitoring-plugins, and linuxfabrik-monitoring-plugins-selinux on RHEL.
• Exit non-zero on unsupported distributions or missing prerequisites.

Delivery:

• Script source under tools/install-monitoring-plugins in this repo.
• Versioned copy deployed to https://repo.linuxfabrik.ch/install-monitoring-plugins.sh via the linuxfabrik.lfops.repo_monitoring_plugins role.
• Publish sha256 checksum alongside; detached GPG signature using the existing Linuxfabrik key.
• Script starts with set -eu -o pipefail, prints a summary before acting, supports DRY_RUN=1.

Docs (INSTALL.md):

• Show both forms, the one-liner and a "download first, review, then run" variant.
• Keep a short paragraph listing what the script actually does.

Out of scope:

• Source tarball install (tracked separately).
• Ansible rollouts (already covered by the LFOps role).