Refactor query transformers in streaming search

Created by: novoselrok

Split from https://github.com/sourcegraph/sourcegraph/pull/25512

The refactor is necessary because we want to replace GQL search suggestions with streaming search suggestions. We do not want to run query transformers on our new queries that we will use for suggestions. My initial approach was to have two streaming search functions: one with transformers and one without transformers. That quickly became unwieldy. So I changed the search functions to receive a query observable instead. The query transformer is applied somewhere up the call chain—this way, the streaming functions do not have to be aware of query transformers and the extension API.