Add AutoFocus

Added auto focus control functions . The code was copied from adafruit_pycamera with minor changes to the timeouts.
The library is reaorganized as a "package" so the ov5640_autofocus.bin file could be included in the package.
There are no "breaking changes".

The usage is the same as for the pycamera library
To execute an autofocus:

cam.autofocus()

to set the focus to a fixed step (0-255)

cam.autofocus_vcm_step=step

new examples have been added. These examples are set up for a Raspberry Pi Pico with the OV5640 PiCowBell breakout board:
ov5640_jpeg_captutre_af.py
ov5640_jpeg_capture_manual_focus.py