feat(generative-ai): Add context caching samples (1)

[gericdong]

Description

Fixes b346529113

Add context caching samples:

• Create context cache

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [generative_ai/context_caching/create_context_cache.py:17](https://github.com/gericdong/edong-python-docs-samples/blob/ccbc7b142e2ea9d737c2fa3303a04dcfdb4f5798/generative_ai/context_caching/create_context_cache.py#L17), tag `generativeaionvertexai_gemini_create_context_cache`

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[m-strzelczyk]

I think #11875 will fix the problem in 3.11 build, but I don't know what's going on in 3.8.