[SDK] Copy SDK related packages to under the pkg/plugin/sdk

[Warashi]

What this PR does:

as title

Why we need it:

They are parts of the SDK, so it's clear to put them under the SDK directory.

Which issue(s) this PR fixes:

Part of #5867

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 60.74332% with 338 lines in your changes missing coverage. Please review.

Project coverage is 28.55%. Comparing base (a6bf12e) to head (24bbf90).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/diff/diff.go	76.99%	54 Missing and 18 partials ⚠️
pkg/plugin/sdk/logpersister/stagelogpersister.go	41.28%	62 Missing and 2 partials ⚠️
.../toolregistry/toolregistrytest/toolregistrytest.go	0.00%	51 Missing ⚠️
pkg/plugin/sdk/logpersister/persister.go	55.55%	48 Missing ⚠️
.../sdk/logpersister/logpersistertest/logpersister.go	0.00%	23 Missing ⚠️
pkg/plugin/sdk/diff/result.go	68.65%	19 Missing and 2 partials ⚠️
pkg/plugin/sdk/pipedapi/client.go	0.00%	20 Missing ⚠️
pkg/plugin/sdk/toolregistry/toolregistry.go	0.00%	15 Missing ⚠️
pkg/plugin/sdk/diff/renderer.go	90.97%	11 Missing and 2 partials ⚠️
pkg/plugin/sdk/signalhandler/handler.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5874      +/-   ##
==========================================
+ Coverage   28.05%   28.55%   +0.50%     
==========================================
  Files         510      520      +10     
  Lines       54420    55284     +864     
==========================================
+ Hits        15267    15787     +520     
- Misses      37997    38317     +320     
- Partials     1156     1180      +24     

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