Supports show origin placement rules

[nolouch]

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.