Kafka if MainConfig is configured, CAP customization will not be set (#1686)

* Fix kafka customize options have been overwirte
and set default options when some options never be setted
options: queue.buffering.max.messages, message.timeout.ms, request.timeout.ms

* simplify the expression

---------

Co-authored-by: xieyidong <xieyidong@laiye.com>

Author: EsonXie

src/DotNetCore.CAP.Kafka/IConnectionPool.Default.cs

@@ -41,11 +41,12 @@ public IProducer<string, byte[]> RentProducer()
         var config = new ProducerConfig(new Dictionary<string, string>(_options.MainConfig))
         {
             BootstrapServers = _options.Servers,
-            QueueBufferingMaxMessages = 10,
-            MessageTimeoutMs = 5000,
-            RequestTimeoutMs = 3000
         };
 
+        config.QueueBufferingMaxMessages ??= 10;
+        config.MessageTimeoutMs ??= 5000;
+        config.RequestTimeoutMs ??= 3000;
+
         producer = BuildProducer(config);
 
         return producer;