*: Add tidb_enable_direct_placement to default forbid direct placement

[lcwangchao]

What problem does this PR solve?

Issue Number: close #31622

Problem Summary:

In #31423 we removed direct placement options from the design of placement rules in SQL. We plan to remove these from code (time permitting), but it is difficult to remove them all at once because they are all thoroughly integrated into tests.

This introduces a new temporary variable to we can prohibit the syntax from users, but allow the current tests to run without breaking. Eventually once all the code is cleaned up and tests are fixed we can remove this variable.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Add `tidb_enable_direct_placement` to default forbid direct placement

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• morgo
• xhebox

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[lcwangchao]

/run-unit-test

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/9d559d0f237cf628ca5348b46c76fed04d97cdf8

[lcwangchao]

/run-mysql-test

[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ea18796

[ti-chi-bot]

@lcwangchao: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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