[ISSUE #5856] Support configurable max-initial-line-length for HTTP request decoder

[hqbhonker]

Description

This PR adds support for configuring the HTTP request decoder properties in ShenYu Gateway, resolving the issue where HTTP requests with long URLs (exceeding 4096 bytes) are rejected.

Changes

shenyu-common

• Added HttpDecoderProperties inner class to NettyHttpProperties with:
  • maxInitialLineLength (default: 4096)
  • maxHeaderSize (default: 8192)
  • maxChunkSize (default: 8192)

shenyu-spring-boot-starter-gateway

• Modified ShenyuNettyWebServerConfiguration to apply httpRequestDecoder settings from configuration
• Added test case for the new httpDecoder properties

shenyu-bootstrap

• Added example configuration in application.yml

Configuration Example

shenyu:
  netty:
    http:
      httpDecoder:
        maxInitialLineLength: 8192
        maxHeaderSize: 16384
        maxChunkSize: 16384

How to Test

1. Configure shenyu.netty.http.httpDecoder.maxInitialLineLength=8192 in application.yml
2. Send an HTTP request with a URL longer than 4096 bytes
3. The request should be processed successfully instead of being rejected

Checklist

• Code follows the project's coding style
• Unit tests added
• Configuration example updated
• Backward compatible (uses Netty default values when not configured)

Closes #5856