[feat] add helm plugin config

[ChristofferNissen]

This pull request introduces significant updates to the helmper project, including the addition of a Helm plugin, installation scripts, and enhancements to the documentation. The changes focus on improving usability, providing installation guidance, and ensuring compatibility with OCI-compliant registries. Below is a breakdown of the most important changes:

Helm Plugin Addition

• Added plugin.yaml to define the Helm plugin, including features like vulnerability scanning, image patching, and artifact signing. It also specifies installation, update, and uninstallation hooks.

Installation and Uninstallation Scripts

• Added scripts/install.sh to automate the installation of the Helm plugin, including checks for prerequisites (cosign, trivy, and copa) and downloading the appropriate binary based on the OS and architecture.
• Added scripts/uninstall.sh to cleanly remove the plugin binary and its directory.

Documentation Enhancements

• Updated README.md to include detailed installation and usage instructions for the Helm plugin, along with prerequisites for specific functionalities. [1] [2] [3]
• Reformatted sections in README.md for better readability and consistency, such as changing headings for installation instructions. [1] [2]

Minor Updates

• Added a disclaimer in README.md to indicate that helmper is in beta and subject to changes.