feat(metadata): MDT posting-block schema foundation for vector index

[chrevanthreddy]

Describe the issue this Pull Request addresses

Adds the Metadata Table (MDT) storage foundation for native vector search proposed in RFC-104 (umbrella #19094): the Avro record types, payload wiring, partition type, and posting-block codec that later PRs (encoder, bootstrap, read path) build on.

Closes #19097

Summary and Changelog

Users gain no behavior change yet; this establishes the on-disk/serde contract for the vector index so subsequent PRs can encode, write, and read it.

Changes (all in hudi-common, additive):

• HoodieMetadata.avsc: seven vector index record types — centroids, quantizer, manifest, cluster stats, posting block, posting delta, tombstone.
• HoodieMetadataPayload: new VectorIndexMetadata field, entry-type constants, and serde for the vector records (co-exists with existing record-index logic).
• MetadataPartitionType.VECTOR_INDEX: new partition type plus record-type wiring.
• HoodieTableMetadataUtil: posting-key → file-group mapping.
• Posting block codec: PostingBlockBuilder / PostingBlockView (structure-of-arrays packing of ~1–4K vectors per record).
• Vector index enums: VectorIndexOptions, VectorIndexType, VectorDistanceMetric.
• MDT raw keys: VectorIndexMetadataKey, VectorPostingPrefixRawKey, VectorClusterRawKey, VectorGenerationManifestRawKey.
• Tests: TestVectorDistanceMetric, TestVectorIndexMetadataKey, TestVectorIndexMetadataPayload (17 cases). Existing HoodieMetadataRecord all-args callers (TestAvroRecordSizeEstimator, TestBufferedRecordSerializer) updated for the appended field.

Impact

Adds one nullable field (VectorIndexMetadata) to the HoodieMetadataRecord Avro schema and a new MetadataPartitionType. The field is optional/defaulted, so existing MDT records and readers are unaffected; the new partition type is inert until a vector index is created (later PRs). No public API removed. No performance impact on existing partitions.

Risk Level

low

Schema change is purely additive (new nullable field + new record types + new partition type), exercised by unit tests; existing metadata read/write paths are untouched apart from the appended constructor arg, which is covered by the updated tests. Full hudi-common compiles and tests pass on current master under JDK17.

Documentation Update

none (user-facing docs land with the feature-complete read/write PRs). The design is documented in RFC-104 (#19291).

Contributor's checklist

• Read through contributor's guide
• Enough context is provided in the sections above
• Adequate tests were added if applicable

[hudi-bot]

CI report:

• 5745d40 UNKNOWN

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