Add gitleaks pre-commit hook for secret detection

Adds gitleaks to .pre-commit-config.yaml to catch secrets before
they are committed. Part of the org-wide rollout (infra#20, infra#43).

Developers need to run 'pre-commit install' after pulling this change.

Author: Sarumyan

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+  - repo: https://github.com/gitleaks/gitleaks
+    rev: v8.30.0
+    hooks:
+      - id: gitleaks