feat: expose `RegionMigrationManagerRef` #20213

Job	Run time
Check (windows-2022)	7m 50s
Check License Header	2m 4s
Check (ubuntu-20.04)	2m 52s
Build GreptimeDB binaries (ubuntu-20.04)	11m 15s
Check typos and docs	13s
Build GreptimeDB binary (profile-CI) (ubuntu-20.04)	11m 43s
Clippy	3m 49s
coverage	14m 23s
Rustfmt	1m 4s
Toml Check	33s
Sqlness Test (Basic)	3m 7s
Fuzz Test (fuzz_create_table)	4m 56s
Sqlness Test (Remote WAL)	3m 34s
Fuzz Test (fuzz_alter_table)	4m 54s
Fuzz Test (fuzz_create_database)	6m 11s
Fuzz Test (fuzz_create_logical_table)	4m 49s
Fuzz Test (fuzz_alter_logical_table)	4m 31s
Fuzz Test (fuzz_insert)	5m 34s
Fuzz Test (fuzz_insert_logical_table)	4m 52s
Unstable Fuzz Test (unstable_fuzz_create_table_standalone)	6m 13s
Fuzz Test with Chaos (Distributed, Remote WAL, fuzz_migrate_mito_regions)	9m 54s
Fuzz Test with Chaos (Distributed, Remote WAL, fuzz_failover_mito_regions)	8m 34s
Fuzz Test (Distributed, Remote WAL, fuzz_create_table)	8m 46s
Fuzz Test with Chaos (Distributed, Remote WAL, fuzz_failover_metric_regions)	8m 6s
Fuzz Test (Distributed, Remote WAL, fuzz_alter_table)	6m 56s
Fuzz Test with Chaos (Distributed, Local WAL, fuzz_migrate_mito_regions)	8m 51s
Fuzz Test (Distributed, Remote WAL, fuzz_create_database)	8m 13s
Fuzz Test (Distributed, Remote WAL, fuzz_create_logical_table)	7m 2s
Fuzz Test (Distributed, Remote WAL, fuzz_alter_logical_table)	7m 29s
Fuzz Test (Distributed, Remote WAL, fuzz_insert)	8m 26s
Fuzz Test (Distributed, Remote WAL, fuzz_insert_logical_table)	8m 0s
	3h 14m 44s

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