[Stage Plugins] Implement ListStageCommands() in pipedapi

[t-kikuc]

What this PR does:

• Implement ListStageCommands() in pipedapi
• Implement ListStageCommands() in commandstore as backend

In the next PR, I'll implement ReportCommandsHandled() to report stage commands as completed in the scheduler.

Why we need it:

Which issue(s) this PR fixes:

Part of #5367
Follows #5521

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 31.81818% with 30 lines in your changes missing coverage. Please review.

Project coverage is 26.46%. Comparing base (4411ffb) to head (4107029).
Report is 53 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/apistore/commandstore/store.go	42.42%	19 Missing ⚠️
pkg/app/pipedv1/cmd/piped/grpcapi/plugin_api.go	0.00%	10 Missing ⚠️
pkg/app/pipedv1/cmd/piped/piped.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5534      +/-   ##
==========================================
+ Coverage   26.43%   26.46%   +0.02%     
==========================================
  Files         465      465              
  Lines       49858    49878      +20     
==========================================
+ Hits        13182    13200      +18     
- Misses      35621    35624       +3     
+ Partials     1055     1054       -1     

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[Warashi]

LGTM