[enh] initial Marginalia.nu support (foss)

Currently it uses a public api_key `/public/` [1]

The 'index' parameter selects the search index, corresponding to the drop down
next to the search field in the main GUI.

    0: popular
    1: blogs
    2: big_sites
    3: default
    4: experimental

'experimental' is more up to date and does not exclude other sites, which is the
case with 'big sites' or 'blogs'.

[1] https://api.marginalia.nu/
[2] https://git.marginalia.nu/marginalia/marginalia.nu
[3] https://news.ycombinator.com/item?id=31536626

Closes: https://github.com/searxng/searxng/issues/1620