Update Dumpling Behavior to not dump policies by default

[morgo]

In a call with the tools team, we discussed the current dumpling behavior of automatically dumping placement policies in dumpling backups. Because dumpling is a logical backup tool and is often used to restore to a destination which has a different physical layout, we agreed to change this. The new behavior will be:

• Dumping placement policies is disabled by default.
• A new option will be added to dumpling which will dump placement policies (TBD). Whether it dumps all placement policies or referenced tables is TBD.
• When the option is enabled, all placement policies will be dumped with CREATE OR REPLACE PLACEMENT POLICY syntax.

This doesn't affect the SHOW CREATE TABLE output which will include the placement policies in the table name. The restore will still need to enable tidb_placement_mode='IGNORE' for a safe restore if the policies don't exist.

[lcwangchao]

It seems that there are no more development works. The current behavior is:

1. Always dump placement policies with annotation format.
2. The placement sql will be dumped as CREATE PLACEMENT POLICY ... without "OR REPLACE" to stay the same with databases and tables. (The lightning will add the 'or replace' clause when importing it)

The above behavior can satisfy the current requirement.