*: split hot_region.go to hot_region.go, hot_region_solver.go and hot_region_solution.go (#9265)

ref #4399

Signed-off-by: okJiang <[email protected]>

Co-authored-by: ti-chi-bot[bot] <108142056+ti-chi-bot[bot]@users.noreply.github.com>

Author: okJiang