docs(genai): Add Provisioned Throughput samples for Gen AI SDK and Vertex AI SDK

[holtskinner]

Description

https://cloud.google.com/vertex-ai/generative-ai/docs/use-provisioned-throughput

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• genai/provisioned_throughput/provisionedthroughput_with_txt.py:17, tag googlegenaisdk_provisionedthroughput_with_txt
• generative_ai/provisioned_throughput/provisioned_throughput_with_txt.py:20, tag generativeaionvertexai_provisioned_throughput_with_txt

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[msampathkumar]

Approved with minor nits. Kindly update the region tags.