vector search: add float16 support

[sivukhin]

Context

We are continue adding compression support and now float16 is on the way!

This PR implements float16 support as a vector column table and vector index compression parameters in the LibSQL.

In current implementation we are basically implement conversion f16 <-> f32 and this way supports all important operations (distance calculation + conversion). This is not that optimal from the performance perspective - but since the main purpose of f16 - is the compression (without much sacrifice in the precision) - I think this is fine to start with.

Reference of the float16 format: https://en.wikipedia.org/wiki/Half-precision_floating-point_format

Changes

• Implement all necessary float16 functions
• Add support float FLOAT16 / F16_BLOB column names
• Add support for compress_neighbors=float16 vector index parameter
• Add test which check that our conversion f16 <-> f32 works the same as reference implementation from python/numpy (see test_libsql_f16.c and test_libsql_f16.py; note that test_libsql_f16.py generates test_libsql_f16_table.h required for the test to work)