feat: Discord integration

[cvauclair]

• [x] I have looked for existing issues (including closed) about this

Feature Request

Add a Discord integration crate to easily create Discord bots from Rig Agents.

Motivation

One of the main use cases of LLMs is to create special purpose chatbots. Although Rig currently provides the building blocks necessary to create chatbots with custom sources of context and tools, Rig is limited in it's support for the deployment of these chatbots. Discord is a popular chat platform used by many communities which supports bots as first class citizens. It is therefore a natural target for deploying Rig agents.

Proposal

Create a new companion crate rig-discord which provides a generic discord bot struct DiscordBot<T: Chat> (name not final). The struct should work for any type that implements the Chat trait defined in rig-core, thus providing an out-of-the-box way for developers to deploy their custom Rig Agents as Discord bots.

Inspiration / reference

rig-core/cli_chatbot

The companion crate should use the serenity crate, which provides full Discord API support in Rust.

Alternatives

None considered at this moment.