Feat/add seed pattern for dfget interface

[wangforthinker]

Ⅰ. Describe what this PR did

Add implementation of dfget interface for seed pattern.

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

[codecov-commenter]

Codecov Report

Merging #1363 into master will decrease coverage by 0.33%. The diff coverage is 43.99%.

@@            Coverage Diff             @@
##           master    #1363      +/-   ##
==========================================
- Coverage   51.26%   50.93%   -0.34%     
==========================================
  Files         135      144       +9     
  Lines        8908     9324     +416     
==========================================
+ Hits         4567     4749     +182     
- Misses       3964     4170     +206     
- Partials      377      405      +28     

Impacted Files	Coverage Δ
dfget/corev2/pattern/seed/downloader.go	0.00% <0.00%> (ø)
dfget/corev2/pattern/seed/manager.go	0.00% <0.00%> (ø)
dfget/corev2/pattern/seed/register.go	0.00% <0.00%> (ø)
dfget/corev2/pattern/seed/report.go	0.00% <0.00%> (ø)
dfget/corev2/pattern/seed/data.go	50.00% <50.00%> (ø)
dfget/corev2/pattern/seed/clientwriter.go	61.03% <61.03%> (ø)
dfget/corev2/pattern/seed/data_scheduler.go	63.55% <63.55%> (ø)
dfget/corev2/pattern/seed/data_map.go	82.35% <82.35%> (ø)
dfget/corev2/pattern/seed/task_state.go	82.75% <82.75%> (ø)
supernode/daemon/mgr/scheduler/manager.go	21.91% <0.00%> (-0.69%)	:arrow_down:
... and 9 more

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