ci: move sbom into it's own file

.github/workflows/sbom.yml

@@ -0,0 +1,17 @@
+name: Generate SBOM
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  GenerateSBOM:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Generate SBOM
+        uses: anchore/sbom-action@v0
+        id: sbom
+        with:
+          image: registry.dangerous.tech/dangeroustech/zerotierbridge:latest
+          registry-username: ${{ secrets.REGISTRY_USERNAME }}
+          registry-password: ${{ secrets.REGISTRY_PASSWORD }}