[Vertex AI] Add FirebaseVertexAIPreview SPM target for preview APIs #878

Job	Run time
spm-package-resolved	1m 14s
pod-lib-lint (ios, macos-14)	6m 33s
sample (macos-13, Xcode_15.2)	4m 28s
sample (macos-14, Xcode_15.4)	2m 38s
spm-unit (macos-13, Xcode_15.2, iOS)	6m 59s
testapp-integration (iOS, macos-15)	3m 19s
spm-unit (macos-14, Xcode_15.4, iOS)	2m 7s
sample (macos-15, Xcode_16)	1m 47s
spm-unit (macos-15, Xcode_16, iOS)	1m 56s
spm-unit (macos-15, Xcode_16, tvOS)	1m 53s
spm-unit (macos-15, Xcode_16, macOS)	2m 31s
spm-unit (macos-15, Xcode_16, watchOS)	2m 14s
spm-unit (macos-15, Xcode_16, catalyst)	1m 50s
spm-unit (macos-15, Xcode_16, visionOS)	2m 42s
	42m 11s

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