Listen on a Unix domain socket

[AlekSi]

There should be a way to configure FerretDB to listen and accept connections on the UNIX domain socket.

Let's add a flag --listen-socket that would contain a single file path to a UNIX socket FerretDB will listen on. Empty value (the default) will disable it. Both --listen-addr and --listen-socket could be given; in that case, FerretDB would listen on both.

• Integration test is in the scope.
• Embedded usage (public ferretdb package) is in the scope.

[Dragomir-Ivanov]

Hello There,
It also work if you make a call to FerretDB API, something like GetEmbedAddress(), and it will return a string containing random(free) listen socket. Then this string could be provided to MongoDB Driver.

Or even a call StartEmbeded(), that will create the listen socket, start listening on, and return its string address, for the Mongo driver to connect,

[AlekSi]

I think that comment belongs to #3