Missing Unix Socket Input plugin docs

[eschabell]

Missing any docs for this input plugin, by @bencetemesvari!

[eschabell]

Research shows the unix socket is used in several input and output plugins, but is not accessible directly:

• forward output plugin uses Unix_Path, to specify the path to a Unix socket to send a Forward message. If set, Upstream is ignored.
• syslog input plugin uses Unix_Perm, If Mode is set to unix_tcp or unix_udp, set the permission of the Unix socket file.
• forward input plugin uses Unix_Path, to specify the path to Unix socket to receive a Forward message. If set, Listen and Port are ignored.
• docker-events input plugin uses Unix_Path, defining the docker socket Unix path.

This is all available by default.

[eschabell]

It's possible to enable it in the build, but this is for unit testing only, not direct usage. See line 62 in the file fluent-bit/cmake /plugins_options.cmake

62 DEFINE_OPTION(FLB_IN_UNIX_SOCKET "Enable Unix socket input plugin" OFF)

[eschabell]

Taking another look at documenting this.

[eschabell]

Ready to merge but requested a code check by @cosmo0920 first.

[eschabell]

See review of attached PR, marking won't fix.