ci: add dependabot integration

Added a `.github/dependabot.yml` configuration file to enable dependabot
for automatically bumping versions of dependencies when possible. This
helps dependencies stay up to date.

Signed-off-by: Ivin Joel Abraham <ivinjabraham@gmail.com>

Author: ivinjabraham

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    open-pull-requests-limit: 10
+    target-branch: "unstable"
+    groups:
+      all-dependencies:
+        applies-to: version-updates
+        patterns:
+          - "*"