add HTTP server-sent-events(SSE) example in brpc http server

[thorneliu]

What problem does this PR solve?

Issue Number:
NA

Problem Summary:
添加了使用brpc 实现HTTP SSE功能的例子，可以用于brpc支持如大语言模型推理服务化等功能

What is changed and the side effects?

Changed:
在example/http_c++中添加了一个HttpSSEServiceImpl实现。
具体而言，server端依次发送以下内容

1. 发送包含如下头部信息的HTTP response

Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive

2. 后续HTTP server基于ProgressiveAttachment向客户段发送SSE的消息，格式：

[field]: value\n

其中， field可以取四个值，它们分别是：data、event、id、retry
Side effects:

• Performance effects(性能影响):
  NO impact
• Breaking backward compatibility(向后兼容性):
  NO compatibiltiy issue

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM