[#6509] Add dependabot configuration (#6510)

### What changes were proposed in this pull request?

Add depedabot configuration to check gravitino and gravitino web UI
dependencies.

### Why are the changes needed?

So we know what to update.

Fix: #6509

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Unable to test locally.

Co-authored-by: Jerry Shao <jerryshao@datastrato.com>

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "gradle"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "npm"
+    directory: "/web/web"
+    schedule:
+      interval: "weekly"