feat: support 'requestTimeout' option

[FredZeng]

No description provided.

[xqq]

You'd better use a more general name, like requestTimeout or networkTimeout, since we are using fetch + ReadableStream on most platforms rather than XHR-based for now.

BTW, what time is your timeout pointing to?
The elapsed time from the request sent until the response header is received?
Or the elapsed time of downloading the full file?

[FredZeng]

I have renamed the xhrTimeout option to requestTimeout, it stands for The elapsed time from the request sent until the response header is received.

Why we need such option?
Because browsers like Chrome require around 20 seconds to throw a timeout error when accessing an unreachable address(e.g. accessing a private network address in public), sometimes we prefer to a faster response.

[xqq]

Have you confirmed that a slow download (where the response header is received) won't cause a timeout in XHR?

[FredZeng]

Thanks for pointing out my mistake. According to the XHR Standard https://xhr.spec.whatwg.org/#the-timeout-attribute, even if a response header is received, a timeout error will be dispatched as long as the request has not been completed, and I have already tested such situation.
So I implemented requestTimeout option using setTimeout instead.