Add GitLab CI template for secret scanning with Gitleaks

[rtrofimenkov-ssdlc]

What's added:

• New gitleaks.gitlab-ci.yml template for automatic secret detection in code
• Support for two scan modes: diff (PR changes only) and full (entire repository)
• Automatic Gitleaks v8.28.0 installation with Linux x64/ARM64 support
• GitLab CI integration via three jobs:
  • gitleaks_diff - for MRs (automatic)
  • gitleaks_full_manual - for manual runs
  • gitleaks_full_scheduled - for scheduled scans
• Custom configuration support via gitleaks.toml
• Detailed results output with links to problematic code locations
• JSON report artifacts for further analysis

Usage:

include:
  - remote: 'https://raw.githubusercontent.com/deckhouse/modules-gitlab-ci/refs/heads/main/templates/gitleaks.gitlab-ci.yml'

This template complements the existing Deckhouse CI template ecosystem, providing additional security for module development.