Scalable: allow user to add customized `balance-region-scheduler`

## Enhancement Task
In big cluster, the balance progress may be too slow because of the slower store has less choice to be picked especially the tiflash . We should allow user add external `balance-region-scheduler ` to balance them, some command like this :

```bash
scheduler add balance-region-scheduler engine tiflash.
success
scheduler show
balance-region-scheduler-engine-tiflash.
```