add an examples app that has a gallery of pre-designed surfaces and allows editing/previewing A2UI JSON

[andrewkolos]

better description TBD

Adds an app to examples that provides the following:

1. The ability to experiment with generating a GenUI surface using the core catalog. This is done with a simple text prompt. (works only if GEMINI_API_KEY is available in the local environment).
2. A curated gallery/catalog of simple surfaces that use the core catalog. Can be useful when evaluating local changes to the styling/implementation of core catalog widgets.
3. The ability to see/edit the backing JSON of surfaces, which (yet to be implemented) will be locally stored for quick future reference.

TBD: whether this app should include a core catalog gallery and replace the existing gallery example.

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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