feat: add observe example using californium server

examples/observe_eclipse.js

@@ -0,0 +1,29 @@
+const coap = require('../') // or coap
+
+const statusRequest = coap.request({
+    method: 'GET',
+    host: 'californium.eclipseprojects.io',
+    pathname: 'obs',
+    observe: true,
+    confirmable: true
+})
+
+let responseCounter = 0
+
+statusRequest.on('response', res => {
+    res.on('error', err => {
+        console.error('Error by receiving: ' + err)
+        this.emit('error', 'Error by receiving: ' + err)
+        res.close()
+    })
+
+    res.on('data', chunk => {
+        console.log(`Server time: ${chunk.toString()}`)
+        responseCounter++
+        if (responseCounter >= 5) {
+            console.log('Successfully received five responses. Closing the ObserveStream.')
+            res.close()
+        }
+    })
+})
+statusRequest.end()