[Vertex AI] Add VertexAI.Preview nested type for preview APIs

[andrewheard]

Added a nested struct Preview in VertexAI for encapsulating Public Preview API surfaces. This clarifies that ImagenModel is in Public Preview.

Before:

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

After:

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

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

#14221
#no-changelog