Send StageIndex to plugins

[Warashi]

What this PR does / why we need it:

We defined the stage_index field in #5229; this PR fills this field.

Which issue(s) this PR fixes:

Follows #5229

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):

[khanhtc1202]

👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.92%. Comparing base (32b33b4) to head (a7447af).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5231      +/-   ##
==========================================
+ Coverage   22.91%   22.92%   +0.01%     
==========================================
  Files         419      419              
  Lines       45321    45322       +1     
==========================================
+ Hits        10385    10392       +7     
+ Misses      34139    34133       -6     
  Partials      797      797              

Flag	Coverage Δ
	22.92% <100.00%> (+0.01%)	⬆️

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