chore: create semgrep.yml (#415)

Creating Semgrep.yml file - Semgrep is a tool that will be used to scan Cloudflare's public repos for Supply chain, code and secrets. This work is part of Application & Product Security team's initiative to onboard Semgrep onto all of Cloudflare's public repos.

In case of any questions, please reach out to "Hrushikesh Deshpande" on cf internal chat.

.github/workflows/semgrep.yml

@@ -0,0 +1,24 @@
+on:
+  pull_request: {}
+  workflow_dispatch: {}
+  push: 
+    branches:
+      - main
+      - master
+  schedule:
+    - cron: '0 0 * * *'
+name: Semgrep config
+jobs:
+  semgrep:
+    name: semgrep/ci
+    runs-on: ubuntu-latest
+    env:
+      SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
+      SEMGREP_URL: https://cloudflare.semgrep.dev
+      SEMGREP_APP_URL: https://cloudflare.semgrep.dev
+      SEMGREP_VERSION_CHECK_URL: https://cloudflare.semgrep.dev/api/check-version
+    container:
+      image: returntocorp/semgrep
+    steps:
+      - uses: actions/checkout@v4
+      - run: semgrep ci