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

[yanminhui]

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What happened + What you expected to happen

yalantinglibs/src/coro_rpc/examples/helloworld/server/main.cpp

Lines 22 to 38 in c75fdff

	int main() {
	// start rpc server
	coro_rpc_server server(std::thread::hardware_concurrency(), 8801);
	// regist normal function for rpc
	server.register_handler<hello_world, A_add_B, hello_with_delay, echo,
	coro_echo>();
	// regist member function for rpc
	HelloService hello_service;
	server
	.register_handler<&HelloService::hello, &HelloService::hello_with_delay>(
	&hello_service);
	// start server
	return server.start() == std::errc{};
	}

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.