feat: add pre-commit configuration (#9)

- add pre-commit-config
- add yamllint config
- add ci/validate-* custom scripts
  - verify no secrets added
  - verify clusters with kustomize and kubeconform
  - verify apps with kustomize and kubeconform

Reviewed-on: #9

.pre-commit-config.yaml

@@ -0,0 +1,53 @@
+repos:
+  # General file checks
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.5.0
+    hooks:
+      - id: check-executables-have-shebangs
+      - id: check-json
+      - id: check-added-large-files
+        args: ['--maxkb=500']
+      - id: check-merge-conflict
+      - id: check-shebang-scripts-are-executable
+      - id: check-symlinks
+      - id: check-toml
+      - id: check-yaml
+        args: [--allow-multiple-documents]
+      - id: detect-aws-credentials
+        args: [--allow-missing-credentials]
+      - id: detect-private-key
+      - id: end-of-file-fixer
+      - id: forbid-new-submodules
+      - id: no-commit-to-branch
+      - id: pretty-format-json
+      - id: trailing-whitespace
+
+  # YAML linting
+  - repo: https://github.com/adrienverge/yamllint.git
+    rev: v1.37.1
+    hooks:
+      - id: yamllint
+        args:
+          [
+            "-d {extends: relaxed, rules: {line-length: disable}, ignore: chart}",
+            "-s",
+          ]
+
+  # Kubernetes manifest validation
+  - repo: local
+    hooks:
+      - id: kubeconform_validate_apps
+        name: kubeconform validate apps
+        entry: ci/validate-apps.sh
+        language: system
+        pass_filenames: false
+      - id: kubeconform_validate_clusters
+        name: kubeconform validate clusters
+        entry: ci/validate-clusters.sh
+        language: system
+        pass_filenames: false
+      - id: no_plain_secrets
+        name: prevent plain kubernetes secrets
+        entry: ci/validate-no-secrets.sh
+        language: system
+        pass_filenames: false