Supports show origin placement rules

## Feature Request

**Is your feature request related to a problem? Please describe:**

Currently, we can only show the placement policy using the following syntax, it's from the tikv meta:

```
SHOW PLACEMENT;
SHOW PLACEMENT FOR DATABASE test;
SHOW PLACEMENT FOR TABLE t1;
```
sometimes, we want to check the origin rules that are true on the pd side to diagnose.

**Describe the feature you'd like:**

add syntax like:

```
SHOW PLACEMENT format='raw' ;
SHOW PLACEMENT FOR DATABASE test format='tidb_json' ;
SHOW PLACEMENT FOR TABLE t1 format='tidb_json' ;
```
which will show the origin rules that get from PD. which is more convenient than using pd-ctl.