ci: set up dependabot configurations (google#123)

This PR adds a `dependabot.yml` to set up dependabot for all gomod
updates, as well as GitHub actions.

.github/dependabot.yml

@@ -0,0 +1,21 @@
+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directories: 
+      - "/api/*"
+      - "/examples/go/*"
+      - "/util/*"
+    schedule:
+      interval: "weekly"
+
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the default location of `.github/workflows`. (You don't need to specify `/.github/workflows` for `directory`. You can use `directory: "/"`.)
+    directory: "/"
+    schedule:
+      interval: "weekly"