Add MDT row families and payload schema for vector indexes

[chrevanthreddy]

Parent: #19094
Related: #18676

Summary

Add MDT row families and payload schema for vector indexes.

Scope

• manifest, centroids, quantizer, generation, cluster, and posting rows
• Avro/model updates
• payload builders and serde
• versioned encoding metadata for multibit support

Acceptance Criteria

• MDT storage model is defined in code
• payload schema supports multibit-aware metadata
• round-trip serde coverage is included