Issue #8035 : Add Data set output transform

[mattcasters]

Adds a Data set output transform so a pipeline can write rows into a unit-test data set (metadata + CSV) from Hop GUI and hop-run, without pointing a Text file output at the data set path.

This addresses #8035. The original request also asked for a “Current rows / All rows” choice on Create data set; that stays out of scope. Canvas Create data set and Write rows to data set already cover interactive capture of the full pipeline stream.

Transform

• Data set name, optional folder ('${HOP_DATASETS_FOLDER}' when empty), and CSV filename
• (Re)create data set from incoming row metadata (default on)
• Validate incoming field count, names (same order, case-insensitive), and types against an existing data set
• Pass-through output; CSV is overwritten each run; multiple copies are rejected
• Empty streams still write a header-only CSV

The dialog is generated from @GuiWidgetElement annotations (GuiCompositeWidgets.addScrolledComposite) with a grouped layout so the fields sit in a fill-and-scroll container above the button bar.

Internals

• New streaming DataSetCsvWriter (Apache VFS); DataSetCsvUtil.writeDataSetData and Write rows to data set use it instead of buffering the whole stream
• Schema helpers on DataSet: createFieldsFromRowMeta / validateRowMeta

Tests and docs

• Unit tests for serialization, schema validation, and CSV round-trip (./mvnw -pl plugins/misc/testing test, apache-rat:check on that module)
• Integration test main-0108-data-set-output (generate then read back against a golden set)
• User-manual page plus nav / data-set / unit-testing docs

GuiWidgetGroupType.BOXES is annotated on the dialog fields; it currently still renders as tabs until #8117.

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