[Firebase AI] Add initial project structure

[andrewheard]

This PR sets up the scaffolding for the Firebase AI SDK structure / test infra. The public API has not yet been updated.

• Split the FirebaseVertexAI folder into FirebaseAI for the new SDK and kept FirebaseVertexAI for backwards compatibility.
• Added new targets in Package.swift and added a FirebaseAI.podspec.
• FirebaseVertexAIUnit keeps VertexAIAPITests, along with the snippets tests, for verifying builds with old API surface.
• The integration tests and remaining unit tests have been moved into FirebaseAI.

#no-changelog

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