[Vertex AI] Add pod to FirebaseManifest

[andrewheard]

Added the FirebaseVertexAI pod to the FirebaseManifest.

Note: Currently needs allowWarnings: true because we emit the following warning on watchOS and tvOS.

firebase-ios-sdk/FirebaseVertexAI/Sources/Constants.swift

Lines 17 to 19 in 413dfef

	#if !os(macOS) && !os(iOS)
	#warning("Only iOS, macOS, and Catalyst targets are currently fully supported.")
	#endif

#no-changelog

[paulb777]

LGTM on green zip build and tests and testing the zip binary instructions in a Vertex AI sample app.

Also, ok to punt this to next milestone since it seems a bit tight.

[andrewheard]

LGTM on green zip build and tests and testing the zip binary instructions in a Vertex AI sample app.

I tested the zip (see #13607) and CocoaPods (see #13609) integrations in our sample app and didn't notice any issues.

Note: Although the version is 11.2-beta, zip and pod support won't be available until the Firebase 11.3 release.