MSC moun/unmout issue

[chegewara]

There is issue with unmout msc device. When device is unmounted from system, but onReady will return true, then tinyusb stack will call onunmout all the time: https://github.com/chegewara/EspTinyUSB/blob/master/src/classes/msc/ramdisk.cpp#L32

We need to think how to implement this:

• add class USBCallbacks in each MSC class (ramdisk, sdcard etc), or
• let the user to implement it in own code.
• implement it in each class and allow user to add its own USBCallbacks, to allow actions in mount/unmount

Pros/cons:

• 1st option - simplify library use, dont bother user with low level actions, but wont let him to add led blinking depending on device state,
• 2nd option - more flexibility, but also more complexity for user,
• 3rd option is best, because will handle onready for user, and allow to add led blinking or similar things depending if device is mounted