feat: Variable splitting supports JSON strings.

[wangliang181230]

What this PR does / why we need it?

feat: Variable splitting supports JSON strings.

Summary of your change

变量拆分：自动将 str 类型的变量转为 obj 类型，避免 JSON path 无法获取变量值。

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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