API import/export/update via files

This MR extends the FormProcessorInstance.export and FormProcessorInstance.export by file parameters, that allow the export to, and the import/update from a json file.

Example:

cvapi FormProcessorInstance.export id=3 to_file=/tmp/my_form_processor.json
cvapi FormProcessorInstance.import file=/tmp/my_form_processor.json

This behaviour does not interfere with the existing functionality; if no file/to_file parameter is given, the behaviour is the same.

This replaces !6 (closed)

mentioned in merge request !6 (closed)

Nice @BjoernE

merged

mentioned in commit 2674c605