Add Dependabot configuration for automated dependency updates

- Configure monthly dependency update checks for Go modules and GitHub Actions
- Set open PR limit to 20 for both ecosystems
- Group AWS SDK packages together to prevent version conflicts
- Group testing-related dependencies for coordinated updates
- Use default Dependabot commit message formatting

Author: orien

.github/dependabot.yml

@@ -0,0 +1,27 @@
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    open-pull-requests-limit: 20
+    groups:
+      aws-sdk:
+        patterns:
+          - "github.com/aws/aws-sdk-go-v2*"
+        update-types:
+          - "minor"
+          - "patch"
+      testing:
+        patterns:
+          - "github.com/stretchr/testify"
+          - "*test*"
+        update-types:
+          - "minor"
+          - "patch"
+
+  - package-ecosystem: "github-actions"
+    directory: "/.github/workflows"
+    schedule:
+      interval: "monthly"
+    open-pull-requests-limit: 20