Added an option to divide by device identifier

[djavan-bertrand]

Change Description:

Added an option to divide attachments by target identifier. Using --identifier will export the attachments into folders named after the device identifier. This option takes precedence over --model and --os, hence these options will be ignored if --identifier is passed. This can be useful if tests are run on two devices with same model and os.

Test Plan/Testing Performed:

Tests have been updated to ensure that this new command works correctly.