Use the "total payload size" field in for data nodes to store file sizes

[limpkin]

• the database model refers to a "(first node) total payload size"
• the child data node doesn't include that field: https://github.com/mooltipass/minible/blob/master/source_code/main_mcu/src/NODEMGMT/nodemgmt.h#L147
• an assert checks that the store data into file message payload has the same size as the data child node
• but somehow the field isn't copied over

We'd therefore need to copy the field and have moolticute display the payload size in MMM by fetching the first child node of a data parent, if the value is != 0