Add Semgrep workflow, CODEOWNERS and Dependabot configuration (#2)

.github/CODEOWNERS

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+*	@auth0-samples/dx-sdks-engineer

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/semgrep.yml

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+name: Semgrep
+
+on:
+  merge_group:
+  workflow_dispatch:
+  pull_request_target:
+    types:
+      - opened
+      - synchronize
+  push:
+    branches:
+      - main
+  schedule:
+    - cron: "30 0 1,15 * *"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
+jobs:
+  authorize:
+    name: Authorize
+    environment: ${{ github.actor != 'dependabot[bot]' && github.event_name == 'pull_request_target' && github.event.pull_request.head.repo.full_name != github.repository && 'external' || 'internal' }}
+    runs-on: ubuntu-latest
+    steps:
+      - run: true
+
+  check:
+    needs: authorize
+
+    name: Check for Vulnerabilities
+    runs-on: ubuntu-latest
+
+    container:
+      image: returntocorp/semgrep
+
+    steps:
+      - if: github.actor == 'dependabot[bot]' || github.event_name == 'merge_group'
+        run: exit 0 # Skip unnecessary test runs for dependabot and merge queues. Artifically flag as successful, as this is a required check for branch protection.
+
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha || github.ref }}
+
+      - run: semgrep ci
+        env:
+          SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}