Use one tool with many subcommands instead of many tools

## Feature Request

### Describe your feature request related problem

We have many tools in https://github.com/tikv/pd/tree/master/tools like `regions-dump`, `stores-dump`, `pd-recover`, `pd-backup`, and so forth. These tools usually help us recover the cluster and investigate the problem we meet.

### Describe the feature you'd like

It's better to use one tool but provide subcommands. We can use cobra to do it. For more details about how to use cobra can be found in https://github.com/spf13/cobra#using-the-cobra-library

The expected results might be:
```
pd-xxx regions-dump
pd-xxx stores-dump
pd-xxx backup
...
```

### Teachability, Documentation, Adoption, Migration Strategy

- Golang
- cobra