[feat] new engine: bt4g added & enabled and disable by default btdigg

Disable btdigg because on most SearXNG instances, SearXNG is blocked by btdigg
due to cloudflare too many requests.

This impementation did not parse the HTML page because there is an API in
XML (RSS).  The RSS feed provides fewer data like amount of seeders/leechers and
the files in the torrent file. It's a tradeoff for a "stable" engine as the XML
from RSS content will change way less than the HTML page.

Closes: https://github.com/searxng/searxng/issues/2553