[server] Balance leaders within each table

[fxbing]

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Description

Description

The current LeaderReplicaDistributionGoal balances leader counts across the whole cluster. However, leaders of an individual table can still be unevenly distributed.

For a hot table, this may cause significant CPU and request-load differences between TabletServers even when the cluster-wide leader count is balanced.

Proposed solution

Make leader balancing table-aware:

• Balance leaders independently within each table when replica placement permits.
• Prefer leadership movement between existing replicas.
• Move replicas when necessary to achieve table-level leader balance, such as during cluster expansion.
• Preserve rack awareness and the existing replica distribution constraints.
• Keep cluster-level leader balancing as a complementary goal.

Willingness to contribute

• I'm willing to submit a PR!

[fxbing]

Hi @swuferhong , could you please associate this issue as a sub-issue of #3867? Thanks!