[load balance] Add load balance framework

[HxpSerein]

Description

This PR adds a region rebalancing feature that allows users to submit a rebalance task through a single Load Balance SQL command.

Automated migration plan

A new cost-aware balancing algorithm computes the optimal region migration plan, which is executed distributedly by the confignode.

Not limited to scaling scenarios

The feature works not only for cluster expansion/shrinking but also for general data skew cases (e.g., uneven region sizes caused by concentrated write load).

Improves cluster balance

Helps address storage imbalance and write-load imbalance across DataNodes, keeping the cluster healthier and more stable.