Add a key for the relay to use for operations #45
Description
The relay needs a key itself to do certain things like make groups via NIP29
I think the best way is just a file in the root directoy called key (ignored from git obviously) and this file is just the raw private key in hex. Users can input their own key or if one does not exist on startup then the relay will use the function to generate a new set and save the file there.
Now the relay has it's own private key and the public can be derived from it.