fix: Ignore unknown fields in rest streaming.

[MarkDaoust]

If the api adds a new field to a streaming response, it shouldn't break old clients.

We found this in the google.ai.generativelanguage API. Colab forces our clients to use rest, so all our streaming examples broke when the API team rolled out a new field.

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Fixes #<issue_number_goes_here> 🦕

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