Feat: Google and Vertex system message mapping

[VisargD]

Gemini models now support systemInstruction param which can be used to send system message. Gateway can map the system message to this new param.

Notes for vertex

Vertex currently fails for gemini 1.5 models when system message is sent because the current config maps system message as a user message and adds an empty model message. This should be fixed if the systemInstruction is mapping is added.

Request body to reproduce this behaviour:

"messages": [
        {
            "role": "system",
            "content": "You are a helpful assistant named Leo"
        },
        {
            "role": "user",
            "content": "Who are you?"
        }
]

Error message:

Unable to submit request because it has an empty text parameter. Add a value to the parameter and try again. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

Notes for google:

For google ai managed API, systemInstruction is only supported from gemini-1.5 model onwards. So older models cannot use the new param. For old models, we can keep send it as a user message like we do now. These are the models which do not support systemInstruction:

const SYSTEM_INSTRUCTION_DISABLED_MODELS = [
  'gemini-1.0-pro',
  'gemini-1.0-pro-001',
  'gemini-1.0-pro-latest',
  'gemini-1.0-pro-vision-latest',
  'gemini-pro',
  'gemini-pro-vision',
];

VertexAI Doc: https://cloud.google.com/vertex-ai/generative-ai/docs/learn/prompts/system-instructions

GoogleAI Doc: https://ai.google.dev/gemini-api/docs/system-instructions#code-generation