Add collections filter to RAG query

New 'collections' property to query request, for example: { “query”: “what is the AI Act?”, “raw_query”: “what is the AI Act?”, “max_characters”: 20000, “chat_id”: “98fca645-8155-4ccf-a035-07fb467ab70e”, “collections”: [“op-cellar","EU case-law”] }

For each entry in collections, find match in sources, collection and reference lists stored in cache and construct corpus query parameters based on each found path. Cache is constructed base on empty aggregations call and stored as 3 separated lists {name, path}.