Add support for PRIMARY_ZONE / ZONES / PRIMARY_RACK / RACKS / PRIMARY_HOST / HOSTS

[morgo]

Enhancement

Part of #18030

In the spec, we include syntactic sugar for the names: REGION, ZONE, RACK, HOST. Currently we've only implemented REGION. Example:

mysql> create placement policy myplacement primary_region="us-east-1" regions="us-east-1";
Query OK, 0 rows affected (0.09 sec)

mysql> alter placement policy myplacement primary_zone="us-east-1a" zones="us-east-1a";
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 47 near "primary_zone="us-east-1a" zones="us-east-1a""