Use "binary vector" (base64) encoding for vectorization results returned by embedding provider

[tatu-at-datastax]

Currently vectorization results (embeddings) are passed as JSON float arrays in responses.
But as we have learned based on performance testing, using Base64 encoding on binary encoded 32-bit floats is significantly more performance (both lower CPU and more compact transfer format). We should see if we can use binary vector encoding for embedding results.

Note: it seems that many embedding providers already include optional settings to return such encoded results so we might not need lots of work to achieve this.