Support for FerretDB client

[LeeBrotherston]

FerretDB is a proxy to replace MongoDB with FerretDB + Postgres, because ewwww MongoDB.

But I wanted to be more lightweight so wanted to replace Postgres with something like postlite in this equation so I can use FerretDB + postlite as a drop in replacement for MongoDB.

I saw you said to raise an issue for clients you'd like so see.... so this is me, raising an issue :)

[AlekSi]

JFYI, we (FerretDB) plan to support SQLite directly: FerretDB/FerretDB#2387