feat: retrieval padding piece

[simlecode]

关联的Issues (Related Issues)

close #360

改动 (Proposed Changes)

附注 (Additional Info)

自查清单 (Checklist)

在你认为本 PR 满足被审阅的标准之前，需要确保 / Before you mark the PR ready for review, please make sure that:

• 符合Venus项目管理规范中关于PR的相关标准 / The PR follows the PR standards set out in the Venus project management guidelines
• 具有清晰明确的commit message / All commits have a clear commit message.
• 包含相关的的测试用例或者不需要新增测试用例 / This PR has tests for new functionality or change in behaviour or not need to add new tests.
• 包含相关的的指南以及文档或者不需要新增文档 / This PR has updated usage guidelines and documentation or not need
• 通过必要的检查项 / All checks are green

[codecov-commenter]

Codecov Report

Merging #491 (b4b4c5e) into master (bcc59df) will increase coverage by 0.22%.
The diff coverage is 50.80%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage   25.18%   25.40%   +0.22%     
==========================================
  Files         177      178       +1     
  Lines       20752    20938     +186     
==========================================
+ Hits         5226     5319      +93     
- Misses      14633    14711      +78     
- Partials      893      908      +15     

Flag	Coverage Δ
unittests	25.40% <50.80%> (+0.22%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
retrievalprovider/httpretrieval/multi_reader.go	62.62% <62.62%> (ø)
retrievalprovider/httpretrieval/server.go	57.08% <37.50%> (-12.10%)	⬇️

... and 1 file with indirect coverage changes

[LinZexiao]

LGTM