Implement vector search

CrateDB supports vector search, there are two components for this feature, the column datatype [FLOAT_VECTOR](https://cratedb.com/docs/crate/reference/en/latest/general/ddl/data-types.html#float-vector) and the [knn_match](https://cratedb.com/docs/crate/reference/en/latest/general/builtins/scalar-functions.html#knn-match-float-vector-float-vector-int) function to perform vector search.

Django does not support vector fields natively, we'd have to create it ourselves.

cc @amotl

Start by reading the linked CrateDB FLOAT_VECTOR and knn_match documentation, then inspect this backend for existing Django field and search-function integration points. Done means Django supports the FLOAT_VECTOR datatype and exposes vector search through knn_match, with the expected behavior covered by tests; the issue does not name specific files or tests.

Written by the indexing model from the issue text.