Support mode setting for PD

## Feature Request

### Describe the feature you'd like

There is a common need to stop/pause PD to do the scheduling. For example, BR and Lightning will pause the PD scheduler and change the PD config temporarily, and unsafe recovery also needs to stop PD scheduling. And right now, we have a feature request to backup/restore on EBS.
We can provide a unified way to control the scheduling. Instead of changing lots of configs, we can only set one config to do it.
Moreover, we can add some scheduling configuration templates to let users choose them according to their different workloads and cluster performance. Also, they can easily change the config back to default if they want.

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy