feat: (Gen AI) Create sample for #get-context-cache-list section

[Valeriy-Burlaka]

Description

Adds a sample for the "Get a list of context caches" section of the context-cache/context-cache-getinfo Vertex AI page.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.8 (see Test Environment Setup)
• 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
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• generative_ai/context_caching/get_list_of_content_caches.py:22, tag generativeaionvertexai_gemini_get_list_of_context_caches

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