Overload operator "<<" for customized_id to support formatting by {fmt}

[empiredan]

Some classes such as rpc_channel are based on template class customized_id. Currently they can only be formatted by {fmt} using to_string(). For example, in src/runtime/rpc/asio_net_provider.cpp, there are codes as below:

    CHECK(channel == RPC_CHANNEL_TCP || channel == RPC_CHANNEL_UDP,
          "invalid given channel {}",
          channel.to_string());

In src/runtime/rpc/network.sim.cpp:

    CHECK(channel == RPC_CHANNEL_TCP || channel == RPC_CHANNEL_UDP,
          "invalid given channel {}",
          channel.to_string());

To simplify the formatting, operator << can just be overloaded which is supported by {fmt}.