Core: Add option to skip column stats in SnapshotChanges

[gtrettenero]

SnapshotChanges reads manifests with no projection and copies added files with
entry.file().copy(), so the per-column statistics maps (column_sizes, value_counts,
null_value_counts, nan_value_counts, lower_bounds, upper_bounds) are always decoded and
retained. Callers that only need file identity — path, size, partition, spec — pay for stats they
never read, which on wide tables dominates the retained heap of the cached file lists. The removed
path already uses copyWithoutStats().

Adds SnapshotChanges.Builder#includeColumnStats(boolean), defaulting to true. When false,
manifests are read with BaseScan.scanColumns(manifest.content()) — the same curated projection
PartitionsTable and PartitionStatsHandler already use for direct manifest reads — and files are
copied without stats. Selecting by manifest content type keeps content, referenced_data_file,
content_offset, content_size_in_bytes and equality_ids on delete files; a single
caller-supplied column list would not be correct for both manifest types.

The default maps to ManifestReader.ALL_COLUMNS, which Schema#select short-circuits, so existing
callers are unaffected. The polarity is inverted relative to Scan#includeColumnStats(), which is
opt-in because scans default to dropping stats.

Testing

Four tests in TestSnapshotChanges cover the default, the opt-out, partition values on a
partitioned table, and content/equalityFieldIds on delete files.

[uros-b]

nan_value_counts is passed as null in the fixture, so testIncludeColumnStatsFalseDropsStats cannot detect a regression where a real non-null nan_value_counts is accidentally retained. Add NAN_VALUE_COUNTS = ImmutableMap.of(3, 1L) to the fixture and assert file.nanValueCounts().isNull() (and assertThat(file.nanValueCounts()).isNull() in the retain-default test). Adding explicit assertions on file_format and content in the same skip-test would further tighten coverage (both survive the projection by construction, but explicit assertions catch future breakage of that invariant).

[gtrettenero]

Added NAN_VALUE_COUNTS to the Metrics fixture with assertions in both the retain-default and skip tests. Also added the file_format and content assertions. The projection check for content is testIncludeColumnStatsFalseRetainsDeleteFileFields, which fails if delete manifests use the data projection.

[vishnuprakaz]

LGTM

[bryanck]

Does this overlap with copyWithoutStats?

[gtrettenero]

ContentFile.copy(boolean) is a default method: withStats ? copy() : copyWithoutStats(). So copy(false) is copyWithoutStats().

The idea for having it is that manifestColumns() delegates to BaseScan.scanColumns() and if a stats column were ever added there for scan-planning reasons, includeColumnStats(false) would silently start returning stats again. But as of today it I remove the boolean then nothing would change since the stats columns are never decoded in the first place when includeColumnStats(false)