[Vertex AI] Add FirebaseVertexAIPreview SPM target for preview APIs #852

Job	Run time
spm-package-resolved	1m 12s
pod-lib-lint (ios, macos-14)	3m 54s
testapp-integration (iOS, macos-15)	4m 31s
spm-unit (macos-13, Xcode_15.2, iOS)	7m 12s
sample (macos-13, Xcode_15.2)	5m 27s
spm-unit (macos-14, Xcode_15.4, iOS)	15m 22s
sample (macos-14, Xcode_15.4)	3m 28s
spm-unit (macos-15, Xcode_16, iOS)	2m 22s
sample (macos-15, Xcode_16)	1m 46s
spm-unit (macos-15, Xcode_16, tvOS)	1m 55s
spm-unit (macos-15, Xcode_16, macOS)	2m 10s
spm-unit (macos-15, Xcode_16, watchOS)	2m 17s
spm-unit (macos-15, Xcode_16, catalyst)	1m 50s
spm-unit (macos-15, Xcode_16, visionOS)	2m 39s
	56m 5s

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