sync-diff-inspector: add hint for check SQL

[joechenrh]

What problem does this PR solve?

Issue Number: close #836

What is changed and how it works?

Add hint /*+ USE_INDEX(...) to force index scan for checksum SQL. Note this index hint is only applied on TiDB source.

The reason we store index columns is to avoid different index names between upstream and downstream databases, even though they have same index columns.

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

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[joechenrh]

/retest

[joechenrh]

/retest