[Vertex AI] Add image generation support using Imagen #860

Job	Run time
pod-lib-lint (ios, macos-14)	43m 25s
spm-package-resolved	1m 28s
testapp-integration (iOS, macos-15)	3m 21s
sample (macos-13, Xcode_15.2)	5m 32s
sample (macos-14, Xcode_15.4)	3m 52s
spm-unit (macos-13, Xcode_15.2, iOS)	4m 36s
sample (macos-15, Xcode_16)	1m 49s
spm-unit (macos-14, Xcode_15.4, iOS)	14m 5s
spm-unit (macos-15, Xcode_16, iOS)	3m 21s
spm-unit (macos-15, Xcode_16, tvOS)	2m 13s
spm-unit (macos-15, Xcode_16, macOS)	1m 50s
spm-unit (macos-15, Xcode_16, watchOS)	2m 13s
spm-unit (macos-15, Xcode_16, catalyst)	1m 51s
spm-unit (macos-15, Xcode_16, visionOS)	3m 5s
	1h 32m 41s

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