apache#21934)

## Which issue does this PR close?

- Closes apache#21928.

## Rationale for this change

ScalarValue::compact copies array data for container types to release
sliced-buffer overhead, but never called .gc() on nested StringViewArray
/ BinaryViewArray. A scalar extracted from a large batch would therefore
still hold a reference to the entire original view backing buffer,
negating the benefit of compaction for any list, struct, or map whose
values are view-typed.
## What changes are included in this PR?

- ScalarValue::compact now compacts nested view buffers for all
container types (List, LargeList, FixedSizeList, ListView,
LargeListView, Struct, Map), trimming StringViewArray / BinaryViewArray
backing buffers to only the
  referenced bytes.
- The internal compact_view_buffers helper recursively handles
FixedSizeList, ListView, LargeListView, and Map.

## Are these changes tested?

new unit tests are added in scalar::tests, one per container type. Each
test verifies that after compact() the backing buffer is reduced to
exactly the bytes of the referenced string, and that the scalar value is
preserved.

## Are there any user-facing changes?

No. The public compact / compacted API is unchanged; this PR only fixes
the behaviour for view-typed nested arrays.

Co-authored-by: Bert Vermeiren <bert.vermeiren@datadobi.com>
Co-authored-by: Dmitrii Blaginin <dmitrii@blaginin.me>
Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>