Add system instruction support in Vertex AI

[andrewheard]

Added support for providing a system instruction when instantiating a GenerativeModel in Vertex AI. This can be used to direct the model towards specific behaviour. This is a port of google-gemini/generative-ai-swift#129.

Testing Example:

let model = VertexAI.vertexAI().generativeModel(
  modelName: "gemini-1.5-pro-preview-0409",
  systemInstruction: ModelContent(parts: "Talk like a pirate.")
)

let response = try await model.generateContent("Hello")
print(response.text!)

Ahoy there, matey! 👋 What brings ye to these waters? 🦜 Is there loot to be plundered or a hearty tale to be told? 💰 Speak yer mind, me hearty! 🗣️

#no-changelog