Add support for system prompts

[simonw]

Released today: https://developers.googleblog.com/2024/04/gemini-15-pro-in-public-preview-with-new-features.html

System instructions: Guide the model's responses with system instructions, now available in Google AI Studio and the Gemini API. Define roles, formats, goals, and rules to steer the model's behavior for your specific use case.

[simonw]

These are implemented in the Python client library now:

• https://github.com/googleapis/google-cloud-python/pull/12533/files

But the REST APi documentation doesn't seem to have been updated yet:

• https://ai.google.dev/api/rest/v1beta/models/streamGenerateContent
• https://ai.google.dev/api/python/google/ai/generativelanguage/GenerateContentRequest

After quite some poking around I found this:

https://github.com/googleapis/google-cloud-python/blob/d9222a97786ce1badae4561410ca5e09386a3377/packages/google-ai-generativelanguage/google/ai/generativelanguage_v1beta/types/generative_service.py#L81-L156

Which included this chunk of code:

    system_instruction: gag_content.Content = proto.Field(
        proto.MESSAGE,
        number=8,
        optional=True,
        message=gag_content.Content,
    )

I found gag_content.Content here: https://github.com/googleapis/google-cloud-python/blob/d9222a97786ce1badae4561410ca5e09386a3377/packages/google-ai-generativelanguage/google/ai/generativelanguage_v1/types/content.py#L32-L59

And after quite a bit of trial and error figured this out as the JSON recipe:

{
  "contents": [
    {
      "role": "user",
      "parts": [
        {
          "text": "say hi three times three different ways"
        }
      ]
    }
  ],
  "systemInstruction": {
    "parts": [
      {
        "text": "in spanish"
      }
    ]
  }
}

[simonw]

$ llm -m p15 'say hi three times three different ways' --system 'in spanish'
¡Hola! 👋 ¡Buenos días! ☀️ ¡Buenas tardes! 😊 

$ llm -m p15 'say hi three times three different ways' --system 'in german' 
Klar, hier sind drei verschiedene Arten, "Hallo" zu sagen:

* Grüß Gott!
* Servus!
* Hallo!