WIP: Serve LFS/attachment with http.ServeContent to Support Range-Request

[6543]

take #18448 and finish it ...

The `ServeData` function now can't handle [Range Request](https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests).
Due to the complexity of this part of HTTP protocol, it is difficult to fully implement it and requires a lot of testing.
The official module `http.ServeContent` has this implementation, and we can replace with it directly.
It can handle `If-Match, If-Unmodified-Since, If-None-Match, If-Modified-Since, If-Range` and more, perfectly!

[silverwind]

Note to self: this is a better version than I have in #20464, will incorporate there.

[6543]

we should use SplitN so go can skip split after first ; ... just some tiny optimization nits

[6543]

@silverwind I'm going to delete the mime stuff as it's unrelated to the pull topic! - so feel free to pick it for your pull :)

[silverwind]

sure, for reference, code was:

func (ct SniffedType) Mime() string {
	return strings.Split(ct.contentType, ";")[0]
}

[silverwind]

If we are talking about micro-optimization, str[:strings.Index(str, ";")] should be even faster than SplitN I think :)

Edit: Done, added it as GetMimeType in #20484.

[silverwind]

Suggested change

	// Mime return the mime
	func (ct SniffedType) Mime() string {
	return strings.Split(ct.contentType, ";")[0]
	}

Delete as discussed, it's unused.

[silverwind]

Not sure this additional function is of any benefit. Cache header can be set like in #20484 via httpcache module.

[wxiaoguang]

-> Make repository response support HTTP range request #24592