Rename PipedServiceClient to PluginServiceClient

[t-kikuc]

What this PR does:

as title

Why we need it:

just for clarification.

If we also want to rename the package pipedservice & pipedapi, let's do it in another PR.

Which issue(s) this PR fixes:

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):

[ffjlabo]

👍

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 25.43%. Comparing base (0a5f895) to head (81e66a3).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/pipedapi/client.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5643      +/-   ##
==========================================
- Coverage   26.68%   25.43%   -1.26%     
==========================================
  Files         477      477              
  Lines       50777    50844      +67     
==========================================
- Hits        13552    12931     -621     
- Misses      36156    36908     +752     
+ Partials     1069     1005      -64     

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[Warashi]

LGTM