tundra.tn:derive and related services - add $prefix? input
parameter to be compatible with WmTN-style bizdoc processing
services.

$prefix? is an optional boolean flag indicating whether to use the
'$' prefix on the standard input arguments (bizdoc, sender, and
receiver) when calling $service. When true $service should implement
the TundraTN/tundra.tn.schema:processor specification, when false
$service should implement the WmTN/wm.tn.rec:ProcessingService
specification. Defaults to true.