Add Security Headers Plugin for OJS 3.3.x

[ashvisualtheme]

This pull request introduces a new generic plugin, "Security Headers by AshVisual Theme," for submission to the Plugin Gallery.

Plugin Summary

This plugin enhances OJS security by adding and managing a comprehensive set of modern HTTP security headers, such as Content-Security-Policy, Strict-Transport-Security, and others. It comes with secure defaults and allows for easy customization through the plugin settings page.

Key Features

• Adds 10 essential HTTP security headers to protect against common web attacks.
• Provides a settings page for administrators to customize each header value.
• Includes automated UI tests (Cypress) to ensure functionality.
• Comes with complete documentation (README) and localization files (en_US, id_ID).

This submission includes the plugin's XML entry for the gallery and is intended to make the plugin available to the wider OJS community.

Plugin Repository: https://github.com/ashvisualtheme/security-headers-plugin

Thank you for your consideration.