DFU spray should convert to UTF8

There is code already in DFU to allow conversion between different UTF versions as the file is sprayed/copied.  Access to these options seems to have been lost when we migrated from dfu to dfuplus.

Since the engines only support reading utf-8 files dfu should default to converting any unicode input (whether csv, json or xml) to utf-8.  There should also be an option to override this just in case it isn't desired (and to also allow despraying to a different encoding).

It probably should not be used when using the option to spray multiple files and concatenate names and lengths since the lengths would then be wrong.

(See FileFormat::restore for details)