refactor: (GenAI) Reorganized Extensions Samples (Group A)

[Thoughtseize1]

Description

Path for all new files is /generative_ai/extensions/

Region Tag	Old file	New File
generativeaionvertexai_delete_extension	gemini_extensions.py	delete_example.py
generativeaionvertexai_create_extension	gemini_extensions.py	create_example.py
generativeaionvertexai_execute_extension	gemini_extensions.py	execute_example.py
generativeaionvertexai_get_extension	gemini_extensions.py	get_example.py
generativeaionvertexai_list_extensions	gemini_extensions.py	list_example.py

❗Files need to be deleted after approval: generative_ai/gemini_extensions.py generative_ai/gemini_extensions_test.py

Checklist

• [x] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [ ] README is updated to include all relevant information
• [ ] 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
• [ ] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 5 region tags.

- [generative_ai/extensions/create_example.py:22](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/f6f175bf97bca5cb58943415198ff43348c40123/generative_ai/extensions/create_example.py#L22), tag `generativeaionvertexai_create_extension` - [generative_ai/extensions/delete_example.py:20](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/f6f175bf97bca5cb58943415198ff43348c40123/generative_ai/extensions/delete_example.py#L20), tag `generativeaionvertexai_delete_extension` - [generative_ai/extensions/execute_example.py:20](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/f6f175bf97bca5cb58943415198ff43348c40123/generative_ai/extensions/execute_example.py#L20), tag `generativeaionvertexai_execute_extension` - [generative_ai/extensions/get_example.py:22](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/f6f175bf97bca5cb58943415198ff43348c40123/generative_ai/extensions/get_example.py#L22), tag `generativeaionvertexai_get_extension` - [generative_ai/extensions/list_example.py:24](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/f6f175bf97bca5cb58943415198ff43348c40123/generative_ai/extensions/list_example.py#L24), tag `generativeaionvertexai_list_extensions`

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