[Vertex AI] Add ImageGenerationResponse for decoding PredictResponse #787

Job	Run time
pod-lib-lint (ios, macos-14)	3m 52s
spm-package-resolved	1m 21s
testapp-integration (iOS, macos-15)	3m 19s
sample (macos-13, Xcode_15.2)	5m 52s
spm-unit (macos-13, Xcode_15.2, iOS)	8m 21s
sample (macos-14, Xcode_15.4)	2m 12s
spm-unit (macos-14, Xcode_15.4, iOS)	2m 18s
sample (macos-15, Xcode_16)	1m 58s
spm-unit (macos-15, Xcode_16, iOS)	2m 8s
spm-unit (macos-15, Xcode_16, tvOS)	1m 55s
spm-unit (macos-15, Xcode_16, macOS)	1m 47s
spm-unit (macos-15, Xcode_16, watchOS)	2m 23s
spm-unit (macos-15, Xcode_16, catalyst)	1m 58s
spm-unit (macos-15, Xcode_16, visionOS)	2m 35s
	41m 59s

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