feat: add Vertex AI for Firebase snippets

[thatfiredev]

These are meant for the Vertex AI for Firebase docs in https://firebase.google.com/docs/vertex-ai/get-started?platform=flutter

They should match the Android ones in https://github.com/firebase/snippets-android/pull/553 and Swift in https://github.com/firebase/snippets-ios/pull/396

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~I had to remove the github_sign_in package in order to add the firebase_vertexai dependency - that package is very old and unmaintained for 3 years, plus it seems like it was not being used in this repo.~ I moved CI changes to #35