feature request: Equivalent of 'sendImageToUser' but for chat rooms

[byteframe]

Changing the 'friend_steamid' property in the request to the commitfileupload endpoint and using chat_group_id and chat_id instead allows one to upload images to chat room groups.

I apologize for maybe not formatting this as a commit request but I struggle with github, and I'm not entirely sure where the code should go seeing as SteamChatClient is in the other module.