Added cdxgen with examples

cdxgen/Dockerfile

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+FROM gcr.io/cloud-builders/npm
+RUN npm install -g @appthreat/cdxgen
+
+ENTRYPOINT ["cdxgen"]

cdxgen/README.md

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+# cdxgen
+
+[cdxgen](https://github.com/AppThreat/cdxgen) creates a valid CycloneDX Software Bill-of-Materials (SBOM) containing an aggregate of all project dependencies for node.js, python, java and golang projects. Optionally, it can submit the generated BOM to [dependency track](https://github.com/DependencyTrack/dependency-track/) or AppThreat server for analysis
+
+## Usage:
+
+Minimal configuration example to generate bom
+
+```yaml
+steps:
+  - name: "gcr.io/$PROJECT_ID/cdxgen"
+    args: ["--output", "bom.xml", "src"]
+```
+
+To generate bom and submit to the server
+
+```yaml
+steps:
+  - name: "gcr.io/$PROJECT_ID/cdxgen"
+    id: "Generate bom.xml and submit to dependency track/AppThreat server"
+    args:
+      [
+        "--output",
+        "bom.xml",
+        "--serverUrl",
+        "https://deptrack.appthreat.io",
+        "--apiKey",
+        "CHANGEME",
+        "src",
+      ]
+```
+
+Follow the [encrypted secrets](https://cloud.google.com/cloud-build/docs/securing-builds/use-encrypted-secrets-credentials) guide to securely store and retrieve the `apiKey` for the server.

cdxgen/cloudbuild.yaml

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+steps:
+  - name: "gcr.io/cloud-builders/docker"
+    args: ["build", "--tag=gcr.io/$PROJECT_ID/cdxgen", "."]
+  - name: "gcr.io/${PROJECT_ID}/cdxgen"
+    args: ["-h"]
+
+images: ["gcr.io/$PROJECT_ID/cdxgen"]
+tags: ["cloud-builders-community"]

cdxgen/examples/cloudbuild.yaml

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+steps:
+  - name: "gcr.io/$PROJECT_ID/cdxgen"
+    id: "Generate bom.xml"
+    args: ["--output", "bom.xml", "src"]
+
+  - name: "gcr.io/$PROJECT_ID/cdxgen"
+    id: "Generate bom.xml and submit to dependency track/AppThreat server"
+    args:
+      [
+        "--output",
+        "bom.xml",
+        "--serverUrl",
+        "https://deptrack.appthreat.io",
+        "--apiKey",
+        "CHANGEME",
+        "src",
+      ]