[OSJC-101] Add support for setting and retrieving additional gear storag...

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@adietish I chose this name explicitly because of the information in the API documentation which is:
'additional_gear_storage	Set additional filesystem storage in gigabytes for the gear profile that the cartridge is running on.'  I confess I don't remember, if I tested it to see if it is truly a 'set' operation which would make sense given I think you would want to be able to lower or raise it depending upon your needs.