Add example plugin implementation using SDK

[Warashi]

What this PR does:

• rename pipedsdk to SDK
  • the name pipedsdk is confusing because it's SDK for plugin not piped.
• add example plugin
• fix error in config unmarshal
• implement some methods for PipelineSyncPlugin

Why we need it:

It's nice that we have example implementation

Which issue(s) this PR fixes:

Part of #5529

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 26.28%. Comparing base (88ec254) to head (90ac919).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/plugin/example/plugin.go	0.00%	10 Missing ⚠️
pkg/plugin/sdk/deployment.go	0.00%	8 Missing ⚠️
pkg/app/pipedv1/plugin/example/main.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5583      +/-   ##
==========================================
- Coverage   26.29%   26.28%   -0.02%     
==========================================
  Files         469      471       +2     
  Lines       50337    50359      +22     
==========================================
  Hits        13235    13235              
- Misses      36041    36063      +22     
  Partials     1061     1061              

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