Transfer leader automatically when receiving SIGTERM

## Feature Request

### Describe your feature request related problem

Currently, during scaling in or upgrade, the deployment tool, like TiUP or TiDB Operator, needs to transfer the PD leader before the scaling in or upgrade, it should be easy for the PD process to transfer the leader if it is the leader when receiving the SIGTERM signal, so that it would be easier for the deployment tool to manage the lifecycle of the PD cluster.
In addition, it would be possible to manage the PD cluster without additional deployment tools.

### Describe the feature you'd like

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy