### What changes were proposed in this pull request?
This PR mainly targets:
1. Expose only explain(mode: String) in Scala side
2. Clean up related codes
- Hide `ExplainMode` under private `execution` package. No particular reason but just because `ExplainUtils` exists there
- Use `case object` + `trait` pattern in `ExplainMode` to look after `ParseMode`.
- Move `Dataset.toExplainString` to `QueryExecution.explainString` to look after `QueryExecution.simpleString`, and deduplicate the codes at `ExplainCommand`.
- Use `ExplainMode` in `ExplainCommand` too.
- Add `explainString` to `PythonSQLUtils` to avoid unexpected test failure of PySpark during refactoring Scala codes side.
### Why are the changes needed?
To minimised exposed APIs, deduplicate, and clean up.
### Does this PR introduce any user-facing change?
`Dataset.explain(mode: ExplainMode)` will be removed (which only exists in master).
### How was this patch tested?
Manually tested and existing tests should cover.
Closes#26898 from HyukjinKwon/SPARK-30200-followup.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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