CI: Add dependabot auto-merge (#15)

CI: Add dependabot

#### Problem

Dependabot is not configured to merge PRs automatically on the repo.

#### Summary of changes

Add dependabot config.

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,17 @@
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.user.login == 'dependabot[bot]' && github.repository_owner == 'solana-program'
+    steps:
+      - name: Enable auto-merge
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}