CI: Add dependabot (#68)

#### Problem

The repo has no dependabot setup.

#### Summary of changes

Similar to other program repos, add dependabot configuration and
auto-approval of PRs.

Author: joncinque

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+updates:
+- package-ecosystem: cargo
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "08:00"
+    timezone: UTC
+  open-pull-requests-limit: 6
+- package-ecosystem: npm
+  directories:
+    - "/"
+    - "/clients/js"
+  schedule:
+    interval: daily
+    time: "09:00"
+    timezone: UTC
+  open-pull-requests-limit: 6

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

@@ -0,0 +1,22 @@
+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: ${{ secrets.GITHUB_TOKEN }}
+      - name: Approve
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}