Add license header workflow

.github/workflows/license_headers.yml

@@ -0,0 +1,21 @@
+name: Check MPL license headers
+
+on:
+  push:
+    branches:
+      - "main"
+  pull_request:
+    branches:
+      - "main"
+  workflow_dispatch:
+
+jobs:
+  check_headers:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install addlicense
+        run: go install github.com/google/addlicense@latest
+      - name: Check headers
+        run: $GOPATH/bin/addlicense -check -l mpl *.rs