Add API to check the unhealth Region, including check the region hole

## Feature Request
In the recovery scenario, we want to get some information to help dianogse:
- [x] Are there exist region holes in the PD region cache, which means the region is missing or not leader https://github.com/tikv/pd/pull/4202
- [ ] Are there exist regions that no leader
- [ ] Are there exist regions that major peers are located in unhealth stores.

We can use some shell command and jq to achieve the goal, but it is not convenient. we can add some API and pd control commands to help find that information.