[core] Add totalFileSize and totalDataFiles to snapshot metadata

[ArnavBalyan]

Purpose

• Add support for total file size and total data file count in snapshot metadata.
• Allows visibility into data files at any snapshot for monitoring table growth and analytics.
• Track at commit time as a running total off the previous snapshot, so reading them later via the snapshots table has no extra overhead.
• Fixes user reported issue [Feature] SnapshotInfo add totalFileSize & totalDataFiles #4997

Tests

• UT

[ArnavBalyan]

cc @JingsongLi @XiaoHongbo-Hope thanks! :)