Use NarClassLoader in protocol hander callbacks.

[Jason918]

Is your enhancement request related to a problem? Please describe.
I am working on a new protocol handler for an existing mq implementation.
ClassNotFoundException occurs when I am calling methods like com.fastjson.JSON.parseObject("some json str", MyConfig.class). This happens because the MyConfig.class is dynamically loaded by a NarClassLoader in protocol hander mechanism, but current thread calling ProtocolHandler::initialize uses the default class load in pulsar.

Describe the solution you'd like
IMHO, Maybe it's not very elegant for protocol handler to handle class loader issue in its own project?

As there is an ProtocolHandlerWithClassLoader object in each protocol handlers, I am suggesting change the thread default class loader to the NarClassLoader before we call interfaces in ProtocolHandler.

Describe alternatives you've considered
Currently, I am getting the NarClassLoader through MyProtocolHandler.class.getClassLoader().
I think it's a bit of tricky.

Additional context
Add any other context or screenshots about the feature request here.

[kaori-seasons]

@Jason918 Do you expect to get the class loader in this way

            if (StringUtils.isNotEmpty(interfaceName)) {
                ClassLoader classLoader = null == Thread.currentThread().getContextClassLoader() ? this.getClass().getClassLoader() : Thread.currentThread().getContextClassLoader();
                Class interfaceClass = Class.forName(interfaceName, true, classLoader);
                FullServiceDefinition fullServiceDefinition = ServiceDefinitionBuilder.buildFullDefinition(interfaceClass, providerUrl.getParameters());
}

see This is how the dubbo2.7 metadata module obtains the class loader
apache/dubbo#4767

[Jason918]

@complone Yes, I am currently using MyProtocolHandler.class.getClassLoader() in my protocol handler project. I am assuming it's better that we set the NarClassLoader as current thread's context class loader. You can see my PR above.