chore(examples): add mqtt example (#523)

Signed-off-by: Xavier Serrano <zombispormedio007@gmail.com>

examples/mqtt-ex/README.md

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+# MQTT Example
+
+The MQTT message protocol are available since v5.3.0
+
+## How To Start
+
+Install and compile:
+
+```bash
+npm install
+npm run compile
+```
+
+Start a MQTT broker using Docker:
+
+```bash
+docker run -it -d  -p 1883:1883 eclipse-mosquitto:2.0 mosquitto -c /mosquitto-no-auth.conf
+```
+
+Then, start
+
+```bash
+npm start
+```

examples/mqtt-ex/package.json

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+{
+  "name": "mqtt-ex",
+  "version": "1.0.0",
+  "description": "Simple mqtt example using CloudEvents types",
+  "repository": "https://github.com/cloudevents/sdk-javascript.git",
+  "main": "build/src/index.js",
+  "types": "build/src/index.d.ts",
+  "files": [
+    "build/src"
+  ],
+  "license": "Apache-2.0",
+  "keywords": [],
+  "scripts": {
+    "start": "node build/index.js",
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "check": "gts check",
+    "clean": "gts clean",
+    "compile": "tsc -p .",
+    "watch": "tsc -p . --watch",
+    "fix": "gts fix",
+    "prepare": "npm run compile",
+    "pretest": "npm run compile",
+    "posttest": "npm run check"
+  },
+  "devDependencies": {
+    "@types/node": "^14.14.10",
+    "@types/ws": "^8.5.4",
+    "gts": "^3.0.3",
+    "typescript": "~4.1.3"
+  },
+  "dependencies": {
+    "cloudevents": "^6.0.3",
+    "mqtt": "^4.3.7"
+  }
+}

examples/mqtt-ex/src/index.ts

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+/* eslint-disable */
+import { CloudEvent, MQTT } from "cloudevents";
+import * as mqtt from "mqtt";
+
+const client = mqtt.connect("mqtt://localhost:1883");
+
+client.on("connect", function () {
+  client.subscribe("presence", function (err) {
+    if (err) return;
+    const event = new CloudEvent({
+      source: "presence",
+      type: "presence.event",
+      datacontenttype: "application/json",
+      data: {
+        hello: "world",
+      },
+    });
+    const { body, headers } = MQTT.binary(event);
+
+    client.publish("presence", JSON.stringify(body), {
+      properties: {
+        userProperties: headers as mqtt.UserProperties,
+      },
+    });
+  });
+});
+
+client.on("message", function (topic, message, packet) {
+  const event = MQTT.toEvent({
+    body: JSON.parse(message.toString()),
+    headers: packet.properties?.userProperties || {},
+  });
+  console.log(event);
+  client.end();
+});

examples/mqtt-ex/tsconfig.json

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+{
+  "extends": "./node_modules/gts/tsconfig-google.json",
+  "compilerOptions": {
+    "rootDir": "./src",
+    "outDir": "./build/",
+    "lib": [
+      "es6",
+      "dom"
+    ]
+  },
+  "include": [
+    "src/**/*.ts",
+    "test/**/*.ts"
+  ],
+  "allowJs": true
+}