Metadata struct for a single daemon declared by an NBPR package.
argv_template is an MFA tuple. The function is invoked as
module.function(validated_opts, opt_flags, *extra_args) and must return
a list of strings (the argv). Defaults to {NBPR.BrPackage, :default_argv, []},
which performs mechanical [flag, value] zipping over the runtime opts.
Packages with subcommand or positional-argv daemons override with their own
MFA.