[Stage Plugins] Add PipedService API to handle commands of SkipStage and ApproveStage

[t-kikuc]

What this PR does:

Added two rpcs of plugin->piped:

• ListStageCommands()
• ReportCommandHandled()

Why we need it:

• To handle SkipStage and ApproveStage actions from UI in Analysis and WaitApproval stage plugins.

cf. See here for the overview of handling SkipStage and ApproveStage:
#5367 (comment)

Which issue(s) this PR fixes:

Part of #5367

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

All modified and coverable lines are covered by tests ✅

Project coverage is 26.43%. Comparing base (6f4d443) to head (0627f12).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5521      +/-   ##
==========================================
- Coverage   26.43%   26.43%   -0.01%     
==========================================
  Files         464      465       +1     
  Lines       49786    49858      +72     
==========================================
+ Hits        13163    13182      +19     
- Misses      35569    35621      +52     
- Partials     1054     1055       +1     

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[Warashi]

LGTM

[ffjlabo]

👍

[t-kikuc]

I updated to remove ReportCommandHandled().

That will be executed in Piped in order to simplify handling commands in plugin-side.

0627f12