Add a configuration key to set workerCount for FileConverter

[n-goncalves]

Since f90636d, the number of workers that can handle file conversion is based on available parallelism or number of CPUs and the config key FileConverter.converter.maxprocesscount that is set to 1 by default.

workersCount = Math.ceil((availableParallelism || numCPUs) * cfgMaxProcessCount);

But someone might not want to use have 1 worker for each core. You can currently achieve that by setting FileConverter.converter.maxprocesscount so that the above calculation reaches the expected value, but it would be clearer to be able to set the worker count using the config file with a new key for instance FileConverter.converter.workercount