* INT-3494: Resolve dir for writing as a Resource
JIRA: https://jira.spring.io/browse/INT-3494
The expression for local directory can be resolved into a `Resource`
or resource location.
* Fix `ExpressionUtils.expressionToFile()` to support `Resource` and
also use `ResourceUtils.getFile(path)` when expression result is a string
* Modify tests to ensure that resource is resolved properly
* Upgrade affected tests to JUnit 5
* Mention an new functionality in docs
* * Improve Java doc for `ExpressionUtils.expressionToFile()`
* Finish the sentence in the `file.adoc`
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