Enhance random nuxeo-importer to add properties

The default random importer implementation create a File document with:

• a random text blob with a configured size in KB + a content filename
• a title equals to the filename

It should be extended to add

• a random title
• additionals dublincore properties
• support empty file (no blob)

The impacts comparing to classic import can be seen here (around 30% decrease in sync throughput):

https://benchmarks.nuxeo.com/misc/index.html#16w23%20NXP-19754%20Test%20import%20with%20random%20dublincore%20properties

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The nuxeo-importer random has a new option withProperties=true that generates random dublincore value creating more realistic random import.