feat(mito2): expose puffin index metadata

[zhongzc]

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Refer to a related PR or issue link (optional)

What's changed and what's your intention?

• add PuffinIndexMetaEntry schema plus record-batch helpers
• traverse puffin blobs to collect bloom/fulltext/inverted metadata
• wire MitoEngine::all_index_metas and cover it with integration tests

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[Copilot]

Pull Request Overview

This PR adds the ability to expose puffin index metadata from the MitoEngine, enabling inspection of various index types including bloom filters, fulltext indexes, and inverted indexes. The implementation provides structured metadata extraction from puffin files and converts it to a queryable format.

Key changes:

• Added PuffinIndexMetaEntry schema and record batch conversion utilities
• Implemented puffin file traversal to collect metadata from different index types (bloom, fulltext, inverted)
• Added MitoEngine::all_index_metas() method with comprehensive integration tests

Reviewed Changes

Copilot reviewed 17 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/store-api/src/sst_entry.rs	Added PuffinIndexMetaEntry struct with schema and record batch conversion
src/mito2/src/engine/puffin_index.rs	New module for collecting index metadata from puffin files
src/mito2/src/engine.rs	Added all_index_metas() method to MitoEngine
src/index/src/target.rs	New IndexTarget enum for encoding/decoding index target keys
src/mito2/src/sst/index/*/creator.rs	Updated to use IndexTarget for consistent target key encoding
src/mito2/src/sst/index/*/applier.rs	Updated to use IndexTarget for consistent target key decoding
src/mito2/src/sst/index/*.rs	Made blob type constants public for metadata collection

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[killme2008]

Almost LGTM