Add a plugin and deriver to expose agents as CKEditor plugins

Problem/Motivation

The AI module has the AI CKEditor submodule which provides a plugin to add AI functionality to CKEditor. Agents give a lot of flexibility for things like iterations and function calls, as well as being configurable via the UI, so it would be great to be able to expose those.

Proposed resolution

Add a plugin with a deriver that exposes AI Agents to be enabled as part of the AI CKEditor plugin.

Remaining tasks

• Build the plugin and deriver
• Consider how we prevent the list becoming huge when there are lots of agents
• Likely a follow up: Allow additional configuration into the agents

User interface changes

N/A

API changes

N/A

Data model changes

N/A