feat: overload stream insertion operator(<<) to support formatting by {fmt}

[empiredan]

This PR is to solve #1220.

The purpose of this PR is to solve some problems in overloading stream insertion operator(<<) to support formatting by {fmt}, including following points:

1. Just use to_string() without converting it to std::string

Some classes, such as gpid and task_code, while implementing their stream insertion operators, have chosen to convert const char * to a temporary std::string object. For example, gpid has implemented as below:

    friend std::ostream &operator<<(std::ostream &os, gpid id)
    {
        return os << std::string(id.to_string());
    }

However, actually since ostream has supported outputting char and const char *, there's no need to converting it again to std::string. According to this answer we can just include <ostream> in the header files.

2. Friend declarations for stream insertion operators

Actually most of stream insertion operators we have implemented are using to_string() which is declared as public. Thus all of them can be declared without friend keyword. However, once the friend keyword are dropped they will have to be defined as non-member template functions and moved out of the definition of the classes. Therefore we can just keep current declarations with friend.

3. Implement stream insertion operator for customized_id

Since some classes such as rpc_channel and network_header_format which have been wrapped based on customized_id support to_string() and are sometimes formatted as output stream, to simplify code we can just implement stream insertion operators for them.

It is noticeable that without implementing stream insertion operator for customized_id the code will be compiled successfully. The reason is that customized_id can be converted to int implicitly by operator int() const for it. However, the formatted information that we really need is the string-typed message; therefore we still need to implement stream insertion operator for customized_id, which is prioritized over operator int() const to be used for formatting by {fmt}.

4. Implement stream insertion operator for threadpool_code

Similar with customized_id, we can also support stream insertion operator for threadpool_code by using to_string() which has been implemented.

Likewise, without implementing stream insertion operator for threadpool_code the code will also be compiled successfully. threadpool_code has implemented operator int() const. Therefore, it's also necessary to implement stream insertion operator for threadpool_code.