[Firebase AI] Add Generable macro for schema generation

[andrewheard]

WIP - Currently only includes scaffolding types, not the macro itself.

Initial Implementation TODOs:

• Add scaffolding for Generable protocols and types that align with Foundation Models.
• Add manually written output that the macro should produce and verify that it compiles.
• Serialize GenerationSchema in JSON Schema format.
• Implement a basic Generable macro that produces the equivalent of the manually written code.
• Implement value(...) methods in GeneratedContent to expose generated values.
• Implement storage and serializing of GenerationGuide values in GenerationSchema.
• Add a Guide "macro" and use it in the Generable macro to handle GenerationGuide annotations.
• Add an equivalent of LanguageModelSession.Response with Gemini-specific additions (e.g., thoughtSummary).

#no-changelog

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