Override System Prompts Without Losing Rulesets

[collindutter]

• I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe.
Users occasionally want to override the system prompt while still persisting Rulesets.

Describe the solution you'd like
For Rulesets to not be lost when overriding the system prompt.

Describe alternatives you've considered
To accomplish this, users need to rely on default_system_template_generator which feels fragile.

def custom_system_template(task: PromptTask) -> str:
    return """
    [context] there are oranges on the table. The forks are on the wall. There is a plant on my head.
    """ + task.default_system_template_generator(task)


task = PromptTask(
    input="{{args[0]}}",
    generate_system_template=custom_system_template,
)

agent = Agent(
    rulesets=[Ruleset(name="language", rules=[Rule("You speak in yoda")])]
)