Extend N1QL expression syntax for vector search

Vector search requests can be used directly by embedding the search request into N1QL's SEARCH() function.

The ask here is to go a little beyond that for on boarding users easily with the a CREATE VECTOR INDEX and a KNN() search function so then can skip coming up to speed on the search index definition syntaxes. 

• Introduce CREATE INDEX capability to build vector indexes in the search space -

  CREATE [VECTOR] INDEX `idx` ON `key-space`({"field": expression, "dimension": constant, "similarity": string}, {"field": expression, "dimension": constant, "similarity": string}) USING FTS;

• Add a KNN function as an added convenience that would spare users the SEARCH() function syntax -

  SELECT * FROM `key-space` WHERE KNN(`key-space`, {object}, {object [optional]})