Add support to send messages to forums 

`@BridgeSixthFormUnion` [has asked](https://discord.com/channels/426894892262752256/426894892262752260/1029706672089870399) for anonymous support for forum channels. I think this would be similar to the work needed to add messages to threads, https://github.com/nminchow/Voltaire/issues/56.

[forums docks](https://discord.com/developers/docs/topics/threads#forums)