Generic Rows-backed GroupColumn so nested GROUP BY keys stay on the column-wise fast path

[zhuqi-lucas]

Part of #22715.

Problem

GroupValuesColumn (the column-wise multi-column group-by path) is all-or-nothing: if a single group-by column has no type-specialized GroupColumn, supported_schema returns false and the entire key falls back to the row-wise GroupValuesRows — even when every other column would have qualified for the column-wise fast path. Nested types (Struct, List, FixedSizeList, ...) currently trigger this, so a mixed key like GROUP BY int_col, struct_col loses the fast path (and its lower memory footprint) entirely because of that one column.

Proposal

(Suggested by @alamb in #23128 (comment).)

Add a single generic GroupColumn backed by arrow's row format (a one-field RowConverter) as a fallback. Then GroupValuesColumn stays usable for any row-encodable type, and row-encoding is confined to just the columns that lack a native builder — instead of dropping the whole key onto GroupValuesRows. Hashing is unchanged (done on the raw input columns, which already supports nested types).

Scope (first step)

The generic RowsGroupColumn framework + routing for the nested types RowConverter can encode (Struct / List / LargeList / FixedSizeList, recursively). A quick local prototype suggests a mixed native + nested key uses roughly half the memory of the all-rows fallback, with grouping identical to the GroupValuesRows reference.

Follow-ups

• Cover types RowConverter cannot encode (e.g. Map, which needs key-order normalization) so supported_schema can always be true.
• With full coverage, retire GroupValuesRows entirely.
• Batch-scoped cache for the scalar equal_to / append_val path (needs a GroupColumn batch hook).

[alamb]

With full coverage, retire GroupValuesRows entirely.

I think this would be really nice -- to be able to remove the whole thing