SearXNG working implementation

[ivan-saorin]

As I saw the discussion about SearXNG integration. I propose this pull request that injects SearXNG results into the knowledge_tool. It adds two env variables: SEARXNG_BASE_URL= SEARXNG_SEARCH_CATEGORIES="general,images:3,videos:3,news,map:3,music:3,science:3,files:3,social_media:3,it:3"

The second allows to configure which search categories are active and how many results one want to receive for each one (default to 5).