refactor(serde): move DataFile JSON handling from REST to core

[kamcheungting-db]

Summary

Move DataFile JSON serialization and deserialization from the REST catalog into core serde.

This is a mechanical move. It does not intentionally change the JSON format or validation behavior. REST keeps forwarding overloads, so existing REST callers continue to use the same API.

Why

Core FileScanTask serde needs to read and write data files without depending on the REST catalog.

Before:

iceberg::rest::DataFileFromJson(json, specs, schema);

After:

iceberg::DataFileFromJson(json, specs, schema);

Stack

This is 1 of 4:

1. This PR: move DataFile serde into core
2. Java-compatible DataFile JSON (fix(serde): make DataFile JSON compatible with Java Iceberg #975)
3. Core FileScanTask JSON serde (feat(serde): support standalone FileScanTask JSON in core #939)
4. Strict schema and partition ID parsing (fix(serde): reject invalid schema and partition IDs in JSON #940)

Each PR targets main; later PRs contain the commits from earlier PRs.

Testing

• Built json_serde_test and rest_catalog_test on the complete stack.
• Full stack: 142 JSON serde tests passed.
• Full stack: 332 REST tests passed; 1 existing test skipped.