[Stage Plugins] Add MetadataKeyStageDisplay and AuthorizedOperators to the SDK

[t-kikuc]

What this PR does:

• Imported MetadataKeyStageDisplay to the SDK
• Enabled to specify AuthorizedOperators in a plugin

I used 922941f 472e02f to use the latest pipe-cd/pipecd.

Why we need it:

• Show stage metadata on UI like Approved by user-1, user-2, Primary: 80%, Canary: 20% flexibly.
• Enable Control-plane validation of approvers for better UX.

Which issue(s) this PR fixes:

Part of #5367

Follows #5980 #5981

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 28.16%. Comparing base (472e02f) to head (619eda2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6000   +/-   ##
=======================================
  Coverage   28.16%   28.16%           
=======================================
  Files         519      519           
  Lines       55937    55938    +1     
=======================================
+ Hits        15756    15757    +1     
  Misses      38931    38931           
  Partials     1250     1250           

Flag	Coverage Δ
.	23.22% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.27% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.33% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	80.26% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	50.92% <100.00%> (+0.02%)	⬆️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[ffjlabo]

LGTM

[Warashi]

👍🏻