zb: Add launchd address support for macOS

Introduce a new "launchd" address option.

It follows D-Bus specification with the same rules (see https://dbus.freedesktop.org/doc/dbus-specification.html#transports-launchd).

env key is required and represents env variable to use for socket path lookup.

Also use the new address in default connect method.

Added a basic test for macOS as well.

Performed a cargo test run on my macOS machine - all tests do pass.

Also added some all(unix, not(target_os="macos")) for stuff that is not applicable or used for macOS.

I also feel like Portability section in README.md could be updated at some point cause there is already a Windows support in the library.