vecindex: add support for vector index prefix columns

[andy-kimball]

The CREATE VECTOR INDEX syntax allows indexing over multiple columns, as long as the vector column to be indexed is the last column in the index definition. The other "prefix" columns can be used to partition the index by tenants, regions, users, etc. The execution engine encodes prefix columns as a byte slice and passes it as a parameter to vector index operations like Insert and Search. While the index itself treats these bytes as an opaque "TreeKey", the CRDB Store implementation incorporates these prefix bytes into KV keys.

This prefixing mechanism has the effect of separating the index into distinct K-means trees, each identified by a unique TreeKey. CRDB partitioning can control where those trees are located, e.g. an app that stores indexed user photo embeddings in a region close to them.

Epic: CRDB-42943

Release note: None

[blathers-crl]

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[cockroach-teamcity]

This change is