[Vertex AI] Add FirebaseVertexAIPreview SPM target for preview APIs

[andrewheard]

Added a FirebaseVertexAIPreview SPM target for Public Preview API surfaces. This clarifies that ImagenModel is in Public Preview. This is an alternative approach to #14334.

Before:

import FirebaseVertexAI

let model = VertexAI.vertexAI().imagenModel("imagen-3.0-generate-001")

After:

import FirebaseVertexAI
import FirebaseVertexAIPreview

let model = VertexAI.vertexAI().imagenModel("imagen-3.0-generate-001")

Note: This is the only way to obtain an instance of ImagenModel.

#14221 #no-changelog