Add chat_history form element

Problem/Motivation

A common thing that we will see more and more is that we want to be able to expose chat history in a nice way in forms. See for instance how its done in platform.openai.com

To make it easier to implement this, we need a form element that is a chat history, that takes common parts of a message like user, content (text/image), tool_id, tool_result.

It will then be represented by a array with associative arrays inside it when expressing default value.

This is already done in the coming ai_agent_tracing module, but it fits better here so it can be reused.

Later a field type could also be added if needed.

Steps to reproduce

Proposed resolution

Add the form elemt chat_history.

Remaining tasks

User interface changes

API changes

Data model changes