[Vertex AI] Make ImageGenerationResponse generic and add image types
#790
| Job | Run time |
|---|---|
| 3m 14s | |
| 1m 7s | |
| 5m 5s | |
| 3m 23s | |
| 2m 6s | |
| 5m 20s | |
| 1m 58s | |
| 2m 38s | |
| 2m 3s | |
| 2m 1s | |
| 1m 53s | |
| 2m 1s | |
| 1m 57s | |
| 3m 6s | |
| 37m 52s |