Add support for customizing the number of workers

[craxal]

At least one user has noted that, despite the AzCopy performance tuning settings Storage Explorer exposes, these are rendered somewhat useless when separate AzCopy activities are started. This is because each activity spawns a separate AzCopy process entirely, and they do not coordinate but rather compete for system resources.

In light of this, it is worth considering making Storage Explorer performance more flexible by providing settings to tune the number of workers/jobs to suit the user's needs.