Sort and organize the cluster configuration read and write commands in pd-ctl

## Enhancement Task

Currently, our pd-ctl contains many commands related to reading and writing cluster-level configurations, but the entry points and behaviors of these commands are not consistent, which makes it difficult for us to organize and compare them during cluster migration or upgrade scenarios. Therefore, the following TODOs are needed for enhancement:

- [ ] Standardize and unify the behavior of the `config show` and `config set` commands.
- [ ] Converge configuration read and write into a unified command, such as `config set`.
- [ ] Provide automation-friendly configuration output and inspection interfaces.