Feat: support multi range

[byene0923]

What type of PR is this?
feat

What this PR does / why we need it (English/Chinese):
en:

• separate singlePart from fsSmallFileReader and bigFileReader
• add bigRangeReader and smallRangeReader, support singlePart and multiPart
• add tests of writeTo() function and case of muliRange

ch:

• 将singlePart 从 fsSmallFileReader 和 bigFileReader 分离
• 添加bigRangeReader 和 smallRangeReader, 支持 singlePart 和 multiPart
• 补充writeTo()方法和muliRange 场景的测试

Which issue(s) this PR fixes:
#93

addtion
ch:
文档：https://yn9nycbt37.feishu.cn/docx/doxcnKSdKp9cMtjtF3ClkN705Qc
问题：

1. bigRangeReader 和 smallRangeReader 存在较多重复代码，想了很久，不太好统一起来- -
2. 如果请求的是目录，标准库针对range场景，是返回全部数据。这点与标准库不一致（原有的singlePart场景也是）

[Duslia]

Has this pr finished? Maybe you can fix ci first.

[byene0923]

Has this pr finished? Maybe you can fix ci first.

yeah, this has been finished, i will fix it this weekend, by the way, i have also pull it to the fasthttp valyala/fasthttp#1398

[byene0923]

finished @Duslia

[codecov]

Codecov Report

Base: 67.25% // Head: 66.28% // Decreases project coverage by -0.96% ⚠️

Coverage data is based on head (3ee0b03) compared to base (d957978).
Patch coverage: 50.22% of modified lines in pull request are covered.

❗ Current head 3ee0b03 differs from pull request most recent head d24b018. Consider uploading reports for the commit d24b018 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #160      +/-   ##
===========================================
- Coverage    67.25%   66.28%   -0.97%     
===========================================
  Files           89       89              
  Lines         8578     8952     +374     
===========================================
+ Hits          5769     5934     +165     
- Misses        2448     2641     +193     
- Partials       361      377      +16     

Impacted Files	Coverage Δ
pkg/app/fs.go	59.81% <49.65%> (-8.85%)	⬇️
pkg/protocol/header.go	73.76% <69.23%> (-0.09%)	⬇️
pkg/app/server/option.go	93.49% <0.00%> (-3.26%)	⬇️

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[CLAassistant]

All committers have signed the CLA.