Add SSE style message framing for streamed responses

[numanelahi]

Commit Message: Add SSE style message framing for streamed responses
Additional Description: This PR adds Server Side Events(SSE) style message framing for the streamed responses transcoded by the gRPC JSON transcoder. Before this the responses were streamed either as JSON array or as NDJSON.
Risk Level: low
Testing: Added unit tests for the feature
Docs Changes: Add details about the new config param for this feature.
Release Notes: Added to the change log.
Platform Specific Features: n/a

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @abeyad
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).
CC @envoyproxy/dependency-shepherds: Your approval is needed for changes made to (bazel/.*repos.*\.bzl)|(bazel/dependency_imports\.bzl)|(api/bazel/.*\.bzl)|(.*/requirements\.txt)|(.*\.patch).
envoyproxy/dependency-shepherds assignee is @moderation

🐱

Caused by: #39325 was opened by numanelahi.

see: more, trace.

[numanelahi]

The Envoy/Prechecks are failing in ./ci/do_ci.sh deps with the following error: aio.core.tasks.exceptions.ConcurrentExecutionError: Download failed https://nvd.nist.gov/feeds/json/cve/1.1/nvdcve-1.1-2022.json.gz: Not Found. This error seems unrelated to the my changes. When I try to download the nvdcve-1.1-2022.json.gz with the same link with out the trailing : it works. The error is probably because of the colon : at the end of the url.

[phlax]

its a very annoying flake - i take full responsibility - but the tool is badly coded and badly cached ... such that if it caches failure you cant easily recache - still worth a try ...

/retest

[abeyad]

/lgtm api

thanks @numanelahi !

[abeyad]

/assign @nezdolik

as grpc_json_transcoder code owner

[moderation]

Updated release date required

[numanelahi]

/retest