Add search model [ENHANCEMENT]

[pandu-k]

Is your feature request related to a problem? Please describe.
There is no way to have a searching model that is different from the indexing model. There is no way to update this searching model.

Describe the solution you'd like
An index now has an update-able search model.

The typical use case is that the search model is a fine-tuned version of the indexing model. This can allow the search experience to adjust relatively quickly as user queries drift over time.

Describe alternatives you've considered
An optional and update-able search model that a user can add. By default the index's model used for indexing is also used for search.

When the search model is specified only the search model is used for searching, and the index's model is used for add_documents calls

Additional context
Add any other context or screenshots about the feature request here.