sync_diff: support json compare

[xiongjiwei]

What problem does this PR solve?

Issue Number: close #606

What is changed and how it works?

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• 3pointer

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[lance6716]

/cc @Leavrth

[Leavrth]

could you add some integration tests?

the script of generating table is here,
https://github.com/pingcap/tidb-tools/blob/master/tests/sync_diff_inspector/run.sh#L20

[Leavrth]

/merge

[ti-chi-bot]

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

Commit hash: 28d99c2