dm: refine openapi table migrate rule source/target

[River2000i]

What problem does this PR solve?

Issue Number: close #4287

What is changed and how it works?

Define TaskTableMigrateRuleSource/TaskTableMigrateRuleTarget as a new type.

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

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If you don't think this PR needs a release note then fill it with `None`.

[ti-chi-bot]

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[ti-chi-bot]

Hi @River2000i. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[lance6716]

/ok-to-test

[River2000i]

/retest

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Files not reviewed (1)

• dm/openapi/spec/dm.yaml: Language not supported