[Vertex AI] Add APIConfig to support switching to the Developer API

[andrewheard]

• Added an APIConfig type to encapsulate the API service (Vertex AI or the Gemini Developer API, see differences), the corresponding endpoint, and the API version (v1 or v1beta).
• Added an internal static function developerAPI(app:apiConfig:) in VertexAI to support switching to the Gemini Developer API for development / integration testing.

Notes:

• These changes are all internal only for now.
• Refactored APIVersion into APIConfig.Version since [FR]: Add support for setting the Vertex AI in Firebase API version #14405 is no longer planned.
• The existing public API surface is tested in the integration tests.
  • The integration tests do not yet cover the Developer API since the backend API differences have not yet been handled.

#no-changelog