Implemented ListStageCommands() on piped side

[t-kikuc]

What this PR does:

as title

Note: Later I'll implement reporting the commands.

Why we need it:

To list stage commands from a plugin.

Which issue(s) this PR fixes:

Follows #5639
Fixes #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

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 25.76%. Comparing base (5079ca7) to head (5e8bdec).

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/apistore/commandstore/store.go	0.00%	13 Missing ⚠️
pkg/app/pipedv1/cmd/piped/grpcapi/plugin_api.go	0.00%	5 Missing ⚠️
pkg/app/pipedv1/cmd/piped/piped.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5685      +/-   ##
==========================================
- Coverage   25.76%   25.76%   -0.01%     
==========================================
  Files         476      476              
  Lines       50976    50981       +5     
==========================================
  Hits        13135    13135              
- Misses      36843    36848       +5     
  Partials      998      998              

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