support changing TABLE placement options through ALTER TABLE

[morgo]

Enhancement

Subtask of #18030

The following should be supported:

mysql> create table t1 (a int);
Query OK, 0 rows affected (0.11 sec)

mysql> create placement policy p1 primary_region="us-east-1" regions="us-east-1";
Query OK, 0 rows affected (0.10 sec)

mysql> alter table t1 placement policy=p1;
ERROR 8200 (HY000): This type of ALTER TABLE is currently unsupported

[morgo]

This seems to be working for partitions:

mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `country` varchar(10) NOT NULL,
  `userdata` varchar(100) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY LIST COLUMNS(country) (
  PARTITION `pEurope` VALUES IN ("DE","FR","GB"),
  PARTITION `pNorthAmerica` VALUES IN ("US","CA","MX")
)
1 row in set (0.00 sec)

mysql> 
mysql> ALTER TABLE t1 PARTITION pNorthAmerica PLACEMENT POLICY=northamerica;
Query OK, 0 rows affected (0.09 sec)

mysql> 
mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `country` varchar(10) NOT NULL,
  `userdata` varchar(100) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
PARTITION BY LIST COLUMNS(country) (
  PARTITION `pEurope` VALUES IN ("DE","FR","GB"),
  PARTITION `pNorthAmerica` VALUES IN ("US","CA","MX") /*T![placement] */ /*T![placement] PLACEMENT POLICY=`northamerica` */
)
1 row in set (0.00 sec)