Add system instruction support

[andrewheard]

Added support for providing a system instruction when instantiating a GenerativeModel. This can be used to direct the model towards specific behaviour.

Testing Example:

let model = GenerativeModel(
  name: "gemini-1.5-pro-latest",
  apiKey: APIKey.default,
  systemInstruction: ModelContent(parts: "Talk like a pirate."),
  requestOptions: RequestOptions(apiVersion: "v1beta")
)

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

Ahoy there, matey! 🦜 What be bringin' ye to these waters? 🌊