add seed downloader

[wangforthinker]

Ⅰ. Describe what this PR did

Add downloader for seed file. Please see refer dragonflyoss/dragonfly#1284.

Ⅱ. Does this pull request fix one issue?

NONE.

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Add ut.

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[pouchrobot]

Thanks for your contribution. 🍻 @wangforthinker
While we thought PR TITLE could be more specific, longer than 20 chars.
Please edit this PR title instead of opening a new one.
More details, please refer to https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md

[codecov-io]

Codecov Report

Merging #1297 into master will increase coverage by 0.01%.
The diff coverage is 54.28%.

@@            Coverage Diff             @@
##           master    #1297      +/-   ##
==========================================
+ Coverage   51.11%   51.12%   +0.01%     
==========================================
  Files         122      124       +2     
  Lines        8082     8152      +70     
==========================================
+ Hits         4131     4168      +37     
- Misses       3622     3646      +24     
- Partials      329      338       +9     

Impacted Files	Coverage Δ
pkg/errortypes/errortypes.go	71.42% <0.00%> (-21.17%)	⬇️
dfdaemon/seed/downloader.go	57.44% <57.44%> (ø)
dfdaemon/seed/utils.go	73.33% <73.33%> (ø)
supernode/daemon/mgr/scheduler/manager.go	21.91% <0.00%> (-0.69%)	⬇️

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[xujihui1985]

LGTM

[lowzj]

The parameter list doesn't contain url and it's initialized in creating instance of a concrete implementation. Why is it designed this way?

And this method has 2 things to do:

1. downloads data from remote server.
2. writes data into target writer.

I think that the step 2 may be not the responsibility of downloader interface. Will it be better to remove it from the interface?

[wangforthinker]

1. a downloader binds to a seed, so the information of seed will be transferred by creating instance of a concrete implementation. In the other word, we may support other protocol like http/https/oss/hdfs ,so the parameters may not be easy to define.
2. in our case, we want to cover the error handler of download. And the data finally should be write ,so we do write in the interface. And some optimization could be make in the interface, such as buffer Write-through which reduces the costs of copying buffer.

[lowzj]

LGTM