[SPARK-3713][SQL] Uses JSON to serialize DataType objects

This PR uses JSON instead of `toString` to serialize `DataType`s. The latter is not only hard to parse but also flaky in many cases.

Since we already write schema information to Parquet metadata in the old style, we have to reserve the old `DataType` parser and ensure downward compatibility. The old parser is now renamed to `CaseClassStringParser` and moved into `object DataType`.

@JoshRosen @davis Please help review PySpark related changes, thanks!
I think this comment is in the wrong place.  We should probably note that this parser is deprecated and is only here for backwards compatibility.  We might even print a warning when it is used so we can get rid of it eventually.