[WIP]Core, Parquet: Carry avg value sizes for v4 content stats

[huan233usc]

Follow-up to #17333.

#17333 collects the average serialized WKB size in FieldMetrics, but ParquetMetrics previously discarded it while assembling Metrics. As a result, the value could not reach a DataFile or be exposed to v4 content_stats adapters.

This change keeps the metric in Iceberg's shared in-memory file model without changing the v1-v3 manifest schema.

• Carry per-field average non-null value sizes through Metrics, ContentFile, file builders, copies, and filtering.
• Populate the map from Parquet field metrics and verify DataWriter to DataFile propagation.
• Expose v4 content stats through the legacy ContentFile view used by manifest readers.
• Preserve compatibility with older Java-serialized Metrics objects.

Scope and dependencies

• This branch is based on main after Parquet: Track avg_value_size_in_bytes for Geo types #17333 and contains only the follow-up commit.
• v1-v3 manifest schemas remain unchanged. In particular, v3 manifests do not persist this optional metric.
• The v4 write-direction content-stats wrapper is being added in Core: V4 write direction wrappers #16936. This PR supplies the ContentFile carrier/accessor that wrapper needs and the v4 read bridge; Core: V4 write direction wrappers #16936 can consume ContentFile.avgValueSizes() after rebasing.

Test Plan

• API serialization, including an old serialized Metrics fixture
• Core content-stats map and tracked-file adapter tests
• Parquet metrics and DataWriter Geo propagation tests
• API, Core, and Parquet RevAPI checks
• API, Core, and Parquet Spotless checks

Verification Commands

./gradlew :iceberg-api:test --tests org.apache.iceberg.TestMetricsSerialization \
  :iceberg-core:test --tests org.apache.iceberg.TestContentStatsBackedMap \
    --tests org.apache.iceberg.TestTrackedFileAdapters \
  :iceberg-data:test \
    --tests org.apache.iceberg.parquet.TestParquetMetrics.testMetricsForGeospatialTypes \
  :iceberg-parquet:test \
    --tests org.apache.iceberg.parquet.TestParquetDataWriter.testGeospatialRoundTrip

./gradlew :iceberg-api:revapi :iceberg-core:revapi :iceberg-parquet:revapi \
  :iceberg-api:spotlessCheck :iceberg-core:spotlessCheck \
  :iceberg-parquet:spotlessCheck

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AI Disclosure

• Model: GPT-5
• Platform/Tool: Codex
• Human Oversight: partially reviewed
• Prompt Summary: Propagate Geo average WKB sizes through Metrics and ContentFile for v4 content stats in a separate follow-up PR.