Add usageMetadata to GenerateContentResponse in Vertex AI

[andrewheard]

Added usageMetadata to GenerateContentResponse.

#no-changelog

[paulb777]

nit: Shouldn't internal line-based comments be two slashes?

[andrewheard]

We don't seem to be very consistent about // vs. /// for internal variables / constants (e.g.,

firebase-ios-sdk/FirebaseVertexAI/Sources/GenerativeModel.swift

Lines 25 to 47 in 3783c8a

	/// The resource name of the model in the backend; has the format "models/model-name".
	let modelResourceName: String
	/// The backing service responsible for sending and receiving model requests to the backend.
	let generativeAIService: GenerativeAIService
	/// Configuration parameters used for the MultiModalModel.
	let generationConfig: GenerationConfig?
	/// The safety settings to be used for prompts.
	let safetySettings: [SafetySetting]?
	/// A list of tools the model may use to generate the next response.
	let tools: [Tool]?
	/// Tool configuration for any `Tool` specified in the request.
	let toolConfig: ToolConfig?
	/// Instructions that direct the model to behave a certain way.
	let systemInstruction: ModelContent?
	/// Configuration parameters for sending requests to the backend.
	let requestOptions: RequestOptions

) but you pointed out a very important issue... these should be public (wasn't caught because the test uses @testable import) and should remain /// for DocC. Adding public now.