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