dependabot: Auto-update tools and GH Actions (#101)

Configure dependabot to auto-update tools dependencies (used for linting
and codegen) and GitHub Actions.

Updating tools regularly should prevent issues like the staticcheck
failure in #100.

Author: abhinav

.github/dependabot.yml

@@ -4,3 +4,13 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+
+  - package-ecosystem: "gomod"
+    directory: "/tools"
+    schedule:
+      interval: "daily"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"