Add support for reading from LittleFS

[ardnew]

• Arduino board:

Adafruit MagTag

• Arduino IDE version (found in Arduino -> About Arduino menu):

arduino-cli Version: 0.30.0 Commit: 83700ca2 Date: 2023-02-09T21:03:27Z

• List the steps to reproduce the problem below (if possible attach a sketch or copy the sketch code in too):

1. Format the internal 4MB flash with an SPIFFS or LittleFS data partition (example partitions.csv)
2. Upload properly-formatted .bmp images to the data partition
3. Try to draw them (e.g., examples/ThinkInkDisplays)

Note this is for the internal QSPI flash interface – the MagTag does not have an SD card reader!

It seems the examples do not handle SPI flash on ESP32 at all (should be using Adafruit_FlashTransport_ESP32 from the Adafruit_SPIFlash library).

I am working on a fix for this issue, but it is requiring a sort of "filesystem" wrapper class to replace the FatVolume and File32 members of Adafruit_ImageReader. Since this is an extensive change, would like some feedback on preferred approach here before continuing or submitting a PR.