[Vertex AI] Add image generation support using Imagen #848

Job	Run time
pod-lib-lint (ios, macos-14)	12m 23s
spm-package-resolved	1m 19s
sample (macos-13, Xcode_15.2)	4m 27s
spm-unit (macos-13, Xcode_15.2, iOS)	6m 36s
sample (macos-14, Xcode_15.4)	3m 3s
testapp-integration (iOS, macos-15)	4m 36s
sample (macos-15, Xcode_16)	2m 59s
spm-unit (macos-14, Xcode_15.4, iOS)	2m 22s
spm-unit (macos-15, Xcode_16, iOS)	3m 20s
spm-unit (macos-15, Xcode_16, tvOS)	3m 18s
spm-unit (macos-15, Xcode_16, macOS)	2m 7s
spm-unit (macos-15, Xcode_16, watchOS)	2m 15s
spm-unit (macos-15, Xcode_16, catalyst)	7m 1s
spm-unit (macos-15, Xcode_16, visionOS)	4m 4s
	59m 50s

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