docs: (core/httpclient): [translate] Done

[DarrenWong]

Checklist

• documentation is changed or added

Affected core subsystem(s)

docs/source/en/core/i18n.md
docs/source/zh-cn/core/i18n.md

Description of change

Translated https://github.com/eggjs/egg/blob/master/docs/source/zh-cn/core/httpclient.md
and minor revise the format of zh-cn:

1. revised the link of #options-参数详解 and a unknown symbol

2. Need further review below example. Don't have the comment in the original files and the example is a little confuse that don't have the comma or semicolon between parameters. Might refer to liburl

### `request` 事件：发生在网络操作发生之前

请求发送之前，会触发一个 `request` 事件，允许对请求做拦截。

```js
app.httpclient.on('request', req => {
  req.url 请求 url
  req.ctx 是发起这次请求的当前上下文

  // 可以在这里设置一些 trace headers，方便全链路跟踪
});

response 事件：发生在网络操作结束之后

请求结束之后会触发一个 response 事件，这样外部就可以订阅这个事件打印日志。

app.httpclient.on('response', result => {
  result.res.status
  result.ctx 是发起这次请求的当前上下文
  result.req 对应的 req 对象，即 request 事件里面那个 req
});

[mention-bot]

@DarrenWong, thanks for your PR! By analyzing the history of the files in this pull request, we identified @atian25, @fengmk2 and @huacnlee to be potential reviewers.

[codecov]

Codecov Report

Merging #1409 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1409   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          29       29           
  Lines         715      715           
=======================================
  Hits          713      713           
  Misses          2        2

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[DarrenWong]

@fengmk2 @atian25 @dead-horse would you mind helping to review?

[fengmk2]

Nice!