add exe backend

add an exe backend with the syntax exe:str|js[,static]:cmd to be able to get secret from command line tools. The secret can be static (always valid and fetched at startup) or by default ephemeral (never valid and fetched only before manifestation).

example

 secrets: {
   "exe:str:s6-clock": "timestamp",
   "exe:str,static:nproc --all": "cpu"
 }

the command is executed as the user defined in the config or as the same user than rconfd or as nobody if root.