Allow option for search prefixes

[do-me]

Some embedding models require prefixes for retrieval like:

• search_query: for https://huggingface.co/nomic-ai/nomic-embed-text-v1.5

Else, retrieval performance degrades.

Should be easy to add just before inferencing. Doesn't need heavy modifications in the current logic.

[do-me]

I believe that with modern models search prefixes like "query: ..." or "document: ..." become obsolete eventually (e.g. that's already the case for some modernBERT-based models). Anyway if it's important to someone, feel free to open a PR.