Align federated buckets.source.toolkit.fluxcd.io observedGeneration semantics with its native

[yike21]

What type of PR is this?
/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes:
part of #4870

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

`karmada-controller-manager`: Mark `.status.observedGeneration` of buckets.source.toolkit.fluxcd.io with `.metadata.generation` only when all members' statuses are algined with its resource template generation.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.23%. Comparing base (d3adcf6) to head (9d4201b).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5193      +/-   ##
==========================================
- Coverage   28.23%   28.23%   -0.01%     
==========================================
  Files         632      632              
  Lines       43712    43712              
==========================================
- Hits        12344    12341       -3     
- Misses      30465    30468       +3     
  Partials      903      903              

Flag	Coverage Δ
unittests	28.23% <ø> (-0.01%)	⬇️

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[XiShanYongYe-Chang]

Thanks a lot~
/lgtm
/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: XiShanYongYe-Chang

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