[bfetch] set 'X-Accel-Buffering':'no' to streaming response headers

[Dosant]

Summary

bfetch streams a list of results back to the end user's browser as soon as possible using HTTP streaming, i.e., it uses Transfer-Encoding: chunked. But it is common for reverse proxies, say Nginx, to (instead of passing through the stream) buffer the stream up to 4 or 8 KiB, and if the request completes within those 4/8 KiB, it instead sets the Content-Length: ... header and sends the whole payload in one go. This is also what is happening in ESE.

This pr addes X-Accel-Buffering: no to streaming response that tells NGINX not to buffer the response. The plan is to make ESE proxy also respect that header and skip buffering the response.

Needed for #139746 (but will actually take effect in ese when cloud's fix lands)
Also would fix streaming responses of all bsearch requests and canvas

Release notes

bfetch response headers now include X-Accel-Buffering: no

[Dosant]

#139532

[vadimkibana]

I think we should merge this anyways, even if it does not help immediately, it is the right header to use here.

[Dosant]

@elasticmachine merge upstream

[dominiqueclarke]

@elasticmachine merge upstream

[Dosant]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 6f09f35
• Cloud Deployment

Metrics [docs]

✅ unchanged

History

• 💚 Build #69197 succeeded 74a3e2f
• 💔 Build #68285 failed 15fce48
• 💔 Build #68264 failed 4677f76
• 💔 Build #68255 failed 8fbe203
• 💚 Build #67938 succeeded fce86b8

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[kibanamachine]

💚 All backports created successfully

Status	Branch	Result
✅	8.4

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