docs(samples): Add prebuilt telecom agent webhook code in python

[yil532]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/webhook_prebuilt_telecom.py:15, tag dialogflow_cx_v3_webhook_prebuilt_telecom

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[parthea]

The failure with Kokoro Prerelease Dependencies test only appears with grpcio==1.49.0rc1 and may be due to grpc/grpc#30640