Last-minute changes to vector search SQL++ syntax

[snej]

As per recent meetings & email threads, for Server SQL++ compatibility.

• There is no more VECTOR_MATCH function.
• VECTOR_DISTANCE renamed to APPROX_VECTOR_DIST.
  • Parameters are (vector expression, target vector, [metric], [nprobes]); last two optional.
  • metric, if given, is a case-insensitive string that must match the index's metric. Current supported values are "EUCLIDEAN_SQUARED", "L2_SQUARED", "COSINE".
  • nprobes, if given, overrides the value specified when the index was created.
• A non-hybrid vector query must have a LIMIT clause. There is no more default limit.
• A hybrid query is one with a WHERE clause (and a APPROX_VECTOR_DIST call, of course.)
• Two new metrics in the API: kC4VectorMetricEuclidean1 and kC4VectorMetricDot.
  • I renamed kC4VectorMetricEuclidean to kC4VectorMetricEuclidean2 to avoid ambiguity.
  • Using the new metrics at runtime requires an updated vector-search extension; see https://github.com/couchbaselabs/mobile-vector-search/pull/69
  • Their SQL++ names are "EUCLIDEAN" (AKA "L2") and "DOT".