feat: add new plugin sse to support Server-Sent Events (SSE)

[billmoling]

Description

Motivation

This PR introduces a new sse plugin for Apache APISIX that enables support for Server-Sent Events (SSE), a mechanism for servers to push updates to the browser over a long-lived HTTP connection.

SSE requires specific proxy settings and headers to ensure streaming behavior works reliably, which are not enabled by default in APISIX. This plugin configures those behaviors for routes or services that serve SSE.

Features

• ✅ Disables response buffering and request buffering to allow real-time data delivery from upstream.
• ✅ Sets a long proxy_read_timeout to prevent SSE connections from being cut off.
• ✅ Sets essential response headers:
  • Content-Type: text/event-stream; charset=utf-8
  • Cache-Control
  • Connection
  • X-Accel-Buffering
• ✅ Supports configuration of proxy_read_timeout, cache_control, and connection_header.
• ✅ Provides optional override of the Content-Type header (enabled by default).
• ✅ Adds debug-level logs to help with observability.

Which issue(s) this PR fixes:

Fixes #10030 #10204

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[SkyeYoung]

Please fix the errors reported by CI.

[billmoling]

Please fix the errors reported by CI.

I fixed the issue from last build. could you please approve another CI build?

[SkyeYoung]

@billmoling There are still some issues with the tests.

You can try testing locally. I'm used to checking these below:

• https://github.com/apache/apisix/blob/master/.github/workflows/build.yml
• https://apisix.apache.org/docs/apisix/building-apisix/#troubleshooting
• https://metacpan.org/pod/Test::Nginx::Socket

[bzp2010]

What's the difference between this and using the response-rewrite plugin to set it up?

[github-actions]

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