feat(shared-shredding): Expose shared-shredding utilities

[dalingmeng]

Purpose

Linked issue: #342

Expose shared-shredding schema metadata utilities for downstream integrations.

This change introduces MapSharedShreddingSchemaUtils as a public schema-level utility for shared-shredding MAP layout. It provides:

• Physical sub-column naming contract, including __field_mapping, __overflow, and __col_i.
• AttachMetadataToSchema to attach serialized shared-shredding metadata to Arrow fields.
• ExtractMetadataFromField to parse shared-shredding metadata from an Arrow field.

The internal MapSharedShreddingDefine continues to own metadata keys, while public callers can reuse the schema contract and metadata read/write helpers without depending on internal src/ headers.

Tests

• MapSharedShreddingSchemaUtilsTest.*

API and Format

This change affects public API in the include dir.

New public header:

• include/paimon/data/shredding/map_shared_shredding_schema_utils.h

New public API:

• MapSharedShreddingSchemaUtils::AttachMetadataToSchema
• MapSharedShreddingSchemaUtils::ExtractMetadataFromField
• MapSharedShreddingSchemaUtils::LogicalToPhysicalSchema

This change does not introduce a new storage format or protocol. It exposes existing shared-shredding schema/metadata conventions through public utilities.

Documentation

No.

Generative AI tooling

Generated-by: Qoder

[lxy-9602]

+1

[zjw1111]

Follow-ups on the newly exposed schema utils. See inline comments for the type-guard and test-gap items.

One doc fix on the PR description:

• The new public header path is include/paimon/data/shredding/map_shared_shredding_schema_utils.h, not include/paimon/shredding/....
• "New public API" omits LogicalToPhysicalSchema, which is also exposed publicly.

Please update the description to match.

[zjw1111]

The description uses close #342, which auto-closes the issue on merge. This PR only exposes the utilities and shouldn't close #342 — please change it to a non-closing reference (e.g. Related to #342).

[zjw1111]

A few non-blocking follow-ups on the new shared-shredding schema utilities. None block merge.

[zjw1111]

+1

[lszskye]

+1