Add SSE style delimiting for message streaming

[numanelahi]

The response_to_json_translator currently supports streaming the messages as JSON array or as new line delimited. This PR adds support for enforcing Server-Sent Events (SSE) message framing (data: <message>\n\n) for streamed messages.

[google-oss-prow]

Hi @numanelahi. Thanks for your PR.

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: numanelahi

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[numanelahi]

/test all

[google-oss-prow]

@numanelahi: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/test all

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