feat: scaffold repartition procedure with plan/resource stubs

[zhongzc]

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Refer to a related PR or issue link (optional)

#6558

What's changed and what's your intention?

• add repartition plan data structures and stubbed procedure skeleton
• wire the procedure into ddl manager with placeholder subprocedure dispatch

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.