[enh] Add autocompleter from Brave

Raw response example: https://search.brave.com/api/suggest?q=how%20to:%20with%20j

Headers are needed in order to get a 200 response, thus Searx user-agent is used.

Other URL param could be  '&rich=false' or  '&rich=true'.

Cherry-pick: https://github.com/allendema/searx/commit/71786bf9cb6fbb175a054692e6951e77769aac1b