Add system prompt functionality in AI agent

[Hooghof]

As a user, I want to be able to configure a system prompt in an Agent component, so that I can give the agent a fixed set of instructions.

Acceptance criteria

1. I can add a System Message / System prompt to an agent component, either inline via a text box or via a reference to a header.
2. the System message is passed on to the LLM's API in the required field.

This is called CamelLangChain4jAgentSystemMessage in the LangChan4j vocabulary.

Jira: https://dovetailworld.atlassian.net/browse/FG-2633