Add Unix Sockets as a alternative to IP for GRPC communication

[hasezoey]

Currently, with the IP way and us defaulting to ipv6, things like #402 can happen. Additionally, we currently dont use any authentication, meaning if somehow the port or IP is exposed to other than "localhost", it could be a problem.
The alternative could to use Unix Sockets (UDS), UDS is already supported by tokio and tonic, like in the tonic examples.
Windows also added support for UDS somewhere in Windows 10, but rust / tokio / tonic currently dont support the windows way yet, making it unix(Linux, BSD, mac) only for now.

I would either propose to add 2 new config options to ComSettings:

• socket which would be the path for the socket
• protocol which would set which mode to use (ip or unix socket)

This way would preserve all options as they are and likely make parsing easier, but could be potentially confusing with many options.

Or changing the existing ComSettings::address to use path name (like /tmp/termusic.sock).
This way would likely lessen confusion, but would make parsing a bit harder and not preserve the other options.

Which of these 2 ways would be better?

[hasezoey]

I didnt get feedback or other suggestions, so i just implemented the least "destructive" way: adding 2 config options:

• protocol to choose which protocol to use (uds or http), and
• socket for the socket path, defaults to SYS_TEMP/termusic.socket (/tmp/termusic.socket)

in #479

Also UDS is for now only available in unix targets.