[coro_rpc] Create the object of service on-demand may be supported

[yanminhui]

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• [x] I searched the issues and found no similar issues.

What happened + What you expected to happen

https://github.com/alibaba/yalantinglibs/blob/c75fdff50b43a153b9f86a50cafea060487e21e7/src/coro_rpc/examples/helloworld/server/main.cpp#L22-L38

Now, create the object of service (e.g. hello_service) before server.start() as pre-condition, but create the object of service is not necessary if service is not called.

It can be supported that allow user creates the object of service while the service is called.

for example, pass one function create() to register_handler().

HelloService* create() 
{
    static HelloService hello_service;
    return &hello_service;
} 

server.register_handler<&HelloService::hello>(create);

Reproduction way

Anything else

Are you willing to submit a PR?

• [ ] Yes I am willing to submit a PR!

[poor-circle]

Lazy evalutation is a good idea, would you like submit a PR? Or I will support this.