feat: add code samples for model optimizer

[yan283]

Add a code sample for model-optimizer using Vertex AI API

Checklist

• [x] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [x] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [x] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

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• generative_ai/text_generation/textgen_model_optimizer_with_txt.py:21, tag generativeaionvertexai_textgen_model_optimizer_with_txt

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[holtskinner]

~We aren't adding new samples for Generative AI with Vertex AI SDK, unless the feature only works with the Vertex AI SDK. Closing this PR as #13296 covers this feature with the Gen AI SDK.~

UPDATE: Not all features work with the Gen AI SDK yet, so we're keeping this PR open.

[holtskinner]

Note: I think that with https://github.com/googleapis/python-genai/releases/tag/v1.11.0 for the Gen AI SDK, all of the missing features for Model Optimizer were added in. So we might not need this PR for the Vertex AI SDK

[msampathkumar]

+1 to Holt's comment.

[msampathkumar]

@yan283 - Looking for your response. Please check reviews added.