fix(spec): preserve manifest extra files

[jianguotian]

Purpose

Recent Paimon Java manifests can own auxiliary files through the nullable _EXTRA_FILES field. Rust must preserve that field when reading and rewriting manifest metadata, otherwise a cross-language rewrite can lose lifecycle ownership of manifest sidecars.

Changes

• Add nullable extra_files to ManifestFileMeta using the Java field name and schema position.
• Preserve the distinction between a legacy missing/null value and an explicitly empty list.
• Include manifest-owned auxiliary files in referenced-file collection and file-size accounting.
• Add Avro round-trip, legacy compatibility, and referenced-file lifecycle tests.

Verification

• Targeted manifest metadata, manifest-list, and referenced-file tests
• cargo clippy -p paimon --lib -- -D warnings

Related Java work

• [core][python] Add extra files to manifest metadata paimon#9746