feat: add proxy-buffering plugin

[AlinsRan]

Summary

The proxy-buffering plugin controls nginx proxy buffering behavior per route. When proxy buffering is disabled, nginx streams responses directly to clients without buffering, which is essential for Server-Sent Events (SSE), streaming APIs, and real-time data delivery.

This plugin runs at a very high priority (21991) so it takes effect before authentication plugins, ensuring the streaming behavior is established early in the request lifecycle.

Changes

• New plugin apisix/plugins/proxy-buffering.lua (priority: 21991)
• Extends apisix/init.lua with a disable_proxy_buffering_access_phase() handler and routing check
• Extends apisix/cli/ngx_tpl.lua with a @disable_proxy_buffering nginx location block (similar to @grpc_pass), wrapped in {% if enabled_plugins["proxy-buffering"] then %}
• CLI test using make init + nginx.conf verification pattern

Example

admin_key=$(yq '.deployment.admin.admin_key[0].key' conf/config.yaml | sed 's/"//g')

# Configure a route with proxy buffering disabled for SSE
curl http://127.0.0.1:9180/apisix/admin/routes/1 -X PUT   -H "X-API-KEY: ${admin_key}"   -d '{
    "uri": "/events",
    "plugins": {
      "proxy-buffering": {
        "disable_proxy_buffering": true
      }
    },
    "upstream": { "type": "roundrobin", "nodes": { "127.0.0.1:1980": 1 } }
  }'

Requests to /events will be proxied with proxy_buffering off, allowing SSE or chunked streaming responses to be delivered immediately to the client.