[Go] [Vertex AI] [Google AI] Add simulated multi-modal input support

[apascal07]

Describe the bug
Controlled here in ModelInfoSupports:

genkit/genkit-tools/common/src/types/model.ts

Line 112 in 96aa4d8

media: z.boolean().optional(),

Example middleware in JS:

genkit/js/ai/src/model/middleware.ts

Line 30 in 96aa4d8

export function downloadRequestMedia(options?: {