Fix several deprecation warnings

Author: benedikt

src/Push.php

@@ -12,6 +12,7 @@ class Push
     public $users;
     public $companies;
     public $events;
+    public $relationships;
 
     public function __construct($config = null)
     {

src/Push/Relation.php

@@ -4,6 +4,7 @@
 class Relation
 {
     private $resourceType;
+    private $client;
 
     public function __construct($resource, $client)
     {

src/Push/Resource.php

@@ -12,6 +12,7 @@ public function __construct($payload)
         $this->payload = $payload;
     }
 
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->payload;

src/Token.php

@@ -6,6 +6,7 @@ class Token
     private $config;
     private $payload;
     private $header;
+    private $key;
 
     public static function generate($identifier, $config = [])
     {