enhance: Extract assign policy from balancer and add StoppingBalancer

[weiliu1031]

issue: #45857

• Create new assign package with unified AssignPolicy interface that abstracts segment and channel assignment logic
• Implement three assign policies: RoundRobin (for fast loading), RowCount-based, and Score-based assignment strategies
• Add AssignPolicyFactory for creating and caching policy instances
• Introduce StoppingBalancer to centralize stopping node balance logic that was previously duplicated across multiple balancers
• Add BalancerFactory for dynamic balancer creation and caching based on configuration
• Refactor existing balancers (ScoreBased, RowCountBased, ChannelLevel) to use the new assign policy abstraction
• Move priority_queue and streaming_query_node_channel_helper to assign package
• Add comprehensive unit tests for all new components
• Add new config parameter for StoppingBalanceAssignPolicy

[sre-ci-robot]

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[mergify]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.73%. Comparing base (4f080bd) to head (8eb630a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #45858   +/-   ##
=======================================
  Coverage   82.73%   82.73%           
=======================================
  Files         524      524           
  Lines       82326    82326           
=======================================
  Hits        68111    68111           
  Misses      14215    14215           

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	82.73% <ø> (ø)
Go	∅ <ø> (∅)

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[sre-ci-robot]

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