[Stage Plugins] Avoid hard coding of Metadata Keys

[t-kikuc]

What this PR does:

• Show the stage metadata by the common key MetadataKey_StageDisplay instead of SkippedBy and ApprovedBy
  • This will be used in k8s plugin too.
• Add AuthorizedOperators in pipelineStage model for Approvers

Why we need it:

• To suppress hard coding of SkippedBy,ApprovedBy, and Approvers
  • It reduces tightly-coupled code of web/server/piped/plugin

TODO:

In the SDK,

• Add pkg/plugin/sdk/constants/constants.go with MetadataKey_StageDisplay
  usage:

import (
   constants "github.com/pipe-cd/piped-plugin-sdk-go/constants"
)

func executeStage(...){
 ...
 client.PutStageMetadata(ctx, constants.MetadataKey_StageDisplay, "Skipped by: user-A")
 ...
}

• Add AuthorizedOperators to PipelineStage struct and toModel()
  usage:

func BuildPipelineSyncStages(...){
	for _, rs := range input.Request.Stages {
		stage := sdk.PipelineStage{
			Index:              rs.Index,
			Name:               rs.Name,
			Rollback:           false,
			Metadata:           map[string]string{},
			AvailableOperation: sdk.ManualOperationApproval,
                        AuthorizedOperators: ["user-1", "user-2"] // Get from the stage config
		}
		stages = append(stages, stage)
	}

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

Attention: Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.

Project coverage is 28.16%. Comparing base (b7555ad) to head (eeadb94).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/datastore/deploymentstore.go	0.00%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5979      +/-   ##
==========================================
- Coverage   28.23%   28.16%   -0.08%     
==========================================
  Files         512      516       +4     
  Lines       55326    55904     +578     
==========================================
+ Hits        15620    15743     +123     
- Misses      38461    38908     +447     
- Partials     1245     1253       +8     

Flag	Coverage Δ
.	23.21% <18.18%> (-0.01%)	⬇️
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	50.89% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (?)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (?)

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[t-kikuc]

Split into #5980 and #5981 for clarity.