Optimize `include_tools` and `system_prompt` config option

[OskarStark]

Follows

• feat: support include tools feature to system prompt #68

Not a fan of having it on the same level like system_prompt, maybe we should be allowed to use sth like:

foo:
    system_prompt: 'My prompt'

and

foo:
    system_prompt:
        prompt: 'My prompt'
        include_tool_definitions: true