[sdk] rename PipelineSyncPlugin to StagePlugin

[t-kikuc]

What this PR does:

as title

Why we need it:

To clarify

Which issue(s) this PR fixes:

Part of #5530

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 2 lines in your changes missing coverage. Please review.

Project coverage is 26.27%. Comparing base (3c5a451) to head (48423e4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/deployment.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5587   +/-   ##
=======================================
  Coverage   26.27%   26.27%           
=======================================
  Files         470      470           
  Lines       50353    50353           
=======================================
  Hits        13230    13230           
  Misses      36062    36062           
  Partials     1061     1061           

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[Warashi]

👍🏻