Support direct placement as table_options

[morgo]

This is a sub-task of Placement Rules in SQL: #18030

This requires the following subtasks:

• *: support new placement rules syntax parser#1299
  parser: Support the following as table_options in CREATE|ALTER TABLE and CREATE|ALTER DATABASE contexts. This should also work for partitions, such as ALTER TABLE t PARTITION p0.

table_option: {
..
 | FOLLOWERS [=] n
 | VOTERS [=] n
 | LEARNERS  [=] n
 | PRIMARY_REGION [=] "str"
 | REGIONS  [=] "str"
 | SCHEDULE [=] "str"
 | CONSTRAINTS  [=] "str"
 | FOLLOWER_CONSTRAINTS  [=] "str"
 | VOTER_CONSTRAINTS  [=] "str"
 | LEARNER_CONSTRAINTS  [=] "str"
 | PLACEMENT POLICY  [=] "str"
}

• After the support of new syntax for parser, the old syntax should be removed.
• Ensure that table_options are persisted in TiDB DDL
• Ensure the database_options for placement are persisted in TiDB DDL
• Ensure the partitionDef options (usually stored as tableOption in ast) for placement are persisted in TiDB DDL
• And visible in SHOW CREATE TABLE but always enclosed in a version specific comment (/*T![placement]). executor: Add support for placement in SHOW CREATE TABLE #27792
• Ensure that table_options respect "Policy Validation" and "Skipping Policy Validation" sections of proposal.

[morgo]

I have moved the tasks of this sub-issue to #18030

This was a suggestion to make it easier to see what tasks are unassigned. It makes sense to me, so I am going to close this issue.