Start a service in the foreground

[danyspin97]

Some services could be needed to start in the foreground but still be supervised by the service manager. For example sway could be managed by tt so it can export environment variables (see #12).

A possible solution is a --foreground flag in tt-start. Example:

tt-start --foreground sway

tt-start could either read the serialized service in a file located in the livedir or get it through a request via the IPC socket.