[CI/CD] Add secrets scanning with Gitleaks

[stepmikhaylov]

Set up Gitleaks secrets scanning in CI. This requires creating the workflow file, configuring a project-specific .gitleaks.toml with allowlist rules appropriate for this codebase, and ensuring the workflow runs on PRs and pushes to develop and main.

Source: #513

[stepmikhaylov]

Done
Gitleaks scan is applied on PRs with checks and commits with lefthook.

[github-actions]