Sending messages with given random id

[Amesaru]

It would be very nice, if you allowed passing random_id parameter to gramers_client::Client::send_message, instead of generating random_id every time.
We want to use it, to be sure we don't send duplicate messages to users, generating random_id from specific parameters and not just a random one

[Lonami]

Feel free to send a PR adding it to InputMessage. And please document the usecase, because I've never seen this explicitly requested for this reason before.