add ability to set lifetime of messages

`time_to_live` parameter
https://firebase.google.com/docs/cloud-messaging/http-server-ref#downstream-http-messages-json

Lifetime of a message
https://firebase.google.com/docs/cloud-messaging/concept-options#lifetime

Author: erlangparasu

src/Fcm.php

@@ -12,6 +12,7 @@ class Fcm
     protected $topic;
     protected $data;
     protected $notification;
+    protected $timeToLive = null;
 
     public function to(array $recipients)
     {
@@ -40,6 +41,13 @@ public function notification(array $notification = [])
 
         return $this;
     }
+    
+    public function timeToLive(int $timeToLive)
+    {
+        $this->timeToLive = $timeToLive;
+        
+        return $this;
+    }
 
     public function send()
     {
@@ -57,6 +65,10 @@ public function send()
         } else {
             $payloads['registration_ids'] = $this->recipients;
         }
+        
+        if ($this->timeToLive !== null && $this->timeToLive >= 0) {
+            $payloads['time_to_live'] = (int) $this->timeToLive;
+        }
 
         $serverKey = config('laravel-fcm.server_key');