sync-diff-inspector: add session configuration in toml

[joechenrh]

What problem does this PR solve?

Issue Number: ref #836, close #844

What is changed and how it works?

Add session variable configuration for each data source.

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]

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[joechenrh]

/release-note-none

[alastori]

Will also close #844

[ti-chi-bot]

@joechenrh: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	985a916	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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