Add Dependabot config (#57)

Since all components in the Heroku Component Inventory are required to have one.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference

GUS-W-17981042.

Author: edmorley

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      rust-dependencies:
+        update-types:
+          - "minor"
+          - "patch"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"