Add index validation of buildPipelineSyncStages in planner

[t-kikuc]

What this PR does:

• Add index validations for duplication and range.
• Add the API spec comments in api.proto and the SDK

Why we need it:

• To prevent errors in the scheduler.
• To make the rollback order determinate
• To keep the rollback spec simple

Which issue(s) this PR fixes:

Part of #5259

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 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 28.49%. Comparing base (275a1e1) to head (221d001).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/controller/planner.go	85.71%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5946      +/-   ##
==========================================
+ Coverage   28.46%   28.49%   +0.03%     
==========================================
  Files         520      520              
  Lines       56402    56438      +36     
==========================================
+ Hits        16056    16084      +28     
- Misses      39080    39087       +7     
- Partials     1266     1267       +1     

Flag	Coverage Δ
.	23.80% <85.71%> (+0.04%)	⬆️
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-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	48.86% <ø> (ø)
.-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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[khanhtc1202]

Thanks 👍

[ffjlabo]

LGTM