Add centralized Snyk security scanning

[fdevans]

This PR adds the centralized Snyk security scanning workflow to enable automated vulnerability detection.

Changes

• Added .github/workflows/snyk-scan.yml workflow
• Configured to run on push and pull requests to main/master branches
• Uses the reusable workflow from rundeck-plugins/.github repository

Benefits

• Automated security vulnerability scanning
• Consistent security configuration across all plugins
• Early detection of security issues

Testing

The workflow will be triggered automatically on:

• Push to main/master branches
• Pull requests to main/master branches
• Manual workflow dispatch

Dependencies

• Requires SNYK_TOKEN and SNYK_ORG_ID secrets to be configured in the repository

[Copilot]

Pull Request Overview

Introduces a reusable Snyk security scanning workflow to automate vulnerability detection across pushes, PRs, and manual dispatches.

• Adds GitHub Actions workflow invoking centralized reusable Snyk scan
• Configures triggers for main/master branches and manual dispatch
• Passes required Snyk secrets to the reusable workflow

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[Copilot]

The workflow does not declare an explicit permissions block, so it will inherit the repository's default GITHUB_TOKEN permissions; explicitly scoping permissions helps follow least-privilege principles. Consider adding a minimal permissions section (e.g., 'contents: read' and 'security-events: write' if the reusable workflow uploads SARIF) above 'jobs:' to reduce risk of unintended token capabilities.