Add getGrantTypesAttribute method (#1705)

Author: gdebrauwer

src/Client.php

@@ -105,6 +105,16 @@ public function tokens()
         return $this->hasMany(Passport::tokenModel(), 'client_id');
     }
 
+    /**
+     * Get the grant types the client can use.
+     *
+     * @return array|null
+     */
+    public function getGrantTypesAttribute()
+    {
+        return $this->attributes['grant_types'] ?? null;
+    }
+
     /**
      * The temporary non-hashed client secret.
      *