docs(generative_ai): Add Vertex AI Express Mode Sample

[holtskinner]

• https://cloud.google.com/vertex-ai/generative-ai/docs/start/express-mode/overview
• NOTE: This cannot be tested in our current infrastructure because Express Mode requires a new project set up with a Gmail account that has never used Google Cloud before

[code-review-assist]

Hi team,

I'm currently reviewing this pull request and will have a full review up in a few minutes. In the meantime, here's a quick summary to get everyone up to speed:

This PR adds a new sample demonstrating how to use Vertex AI's Express Mode with the Gemini model. The intent is to provide a clear, concise example of using the API key approach for generating content. The changes include:

• A new Python file (generative_ai/express_mode/api_key_example.py) containing the sample code. This code shows how to initialize the Vertex AI client using an API key, create a GenerativeModel instance, and generate text using the generate_content method.
• Updated requirements.txt and requirements-test.txt to include necessary dependencies.
• A new noxfile_config.py file was added to configure testing. This file sets up the testing environment, specifying ignored Python versions and other testing configurations.

The author notes that this sample cannot be fully tested within the current infrastructure due to Express Mode's requirements for a new project and a fresh Gmail account. I'll keep this limitation in mind during my review.

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[code-review-assist]

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[snippet-bot]

Here is the summary of changes.

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• generative_ai/express_mode/api_key_example.py:18, tag generativeaionvertexai_gemini_express_mode

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