feat: adds vector index meta

[killme2008]

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What's changed and what's your intention?

As the title said

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[gemini-code-assist]

Summary of Changes

Hello @killme2008, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces the foundational Protocol Buffer definitions for managing vector index metadata and statistics. By defining VectorIndexMeta and VectorIndexStats, it provides a standardized and extensible way to describe and track vector indices. The changes also include the necessary code generation for C++, Go, and Java, and integrate the new protobufs into the build system, laying the groundwork for advanced vector indexing capabilities.

Highlights

• New Protobuf Definition: Introduced vector_index.proto to define new data structures: VectorIndexMeta for storing metadata about vector indices and VectorIndexStats for capturing statistics.
• Vector Index Metadata Fields: VectorIndexMeta includes fields such as the index engine type (VectorIndexEngine), vector dimension (dim), distance metric (VectorDistanceMetric), HNSW parameters (connectivity, expansion factors for add and search), and size information (null bitmap size, index size).
• Vector Index Statistics: VectorIndexStats provides metrics like total row count, indexed row count, and null vector count, offering insights into the index's composition.
• Multi-language Code Generation: Generated corresponding C++, Go, and Java source files from the new protobuf definition, ensuring compatibility and usability across different parts of the system.
• Build System Integration: The build.rs file was updated to include the new vector_index.proto in the protobuf compilation process, integrating it into the project's build pipeline.

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[gemini-code-assist]

Code Review

This pull request introduces the protobuf definitions for vector index metadata. The changes are well-contained and primarily involve adding a new .proto file and its corresponding generated code across different languages (Rust, C++, Go, Java). My review focuses on the new protobuf definition file, as the others are auto-generated. I've suggested a couple of improvements to the enums to align with Protobuf best practices, specifically regarding the use of _UNSPECIFIED as the zero value to prevent unintended default behavior.