CI: Add dependabot and auto-merge job (#64)

#### Problem

The repo has working CI, but no dependabot support.

#### Summary of changes

Add dependabot configuration and an auto-merge job to land changes
automatically, similar to all the other repos.

Author: joncinque

.github/dependabot.yml

@@ -0,0 +1,21 @@
+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+- package-ecosystem: cargo
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "08:00"
+    timezone: UTC
+  open-pull-requests-limit: 6
+- package-ecosystem: npm
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "09:00"
+    timezone: UTC
+  open-pull-requests-limit: 6

.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 }}