Making visible the USB_SendStringDescriptor() via USB API

Some use cases require custom strings to be sent from the device to the host (in example, OEM Manufacturer, battery chemistry, and so on). The function for that already exists in the USBCore.cpp but it is not exposed in the USB API.

Start in USBCore.cpp at USB_SendStringDescriptor() and inspect how the existing function relates to the public USB API. Expose that function through the API so callers can provide custom device strings, then verify that the function is accessible and can send those descriptors to the host.

Written by the indexing model from the issue text.