Add support for MJPEG on UVC

[kkitayam]

Describe the PR
Add support for MJPEG on UVC.

It is tested with video_capture example on frdm_kl25z and raspberry pi pico

Additional context
video_capture uses MJPG video instead of uncompressed video when it builds with CFG_EXAMPLE_VIDEO_READONLY defined.

If a MCU has not enough SRAM for frame buffers, CFG_EXAMPLE_VIDEO_READONLY is already defined in board.mk at hw/bsp/<board>. For example, frdm_kl25z and so on. Any other MCU which has enough SRAM for frame buffers use uncompressed video in default. To force the use of MJPG video, add the FORCE_READONLY argument to make or cmake.
For example:

make FORCE_READONLY=1 BOARD=frdm_kl25z
cmake -B build_rp2040 -G Ninja -DFAMILY=rp2040 -DPICO_SDK_PATH=c:\pico_sdk -DFORCE_READONLY=1

[kkitayam]

@me-no-dev

Would you see if this PR is useful or not for your application which indicated #1118 (comment) ?

[tswan-quasi]

I only checked the descriptors and class files as that is what I used in my setup. Some type consistency changes that also affect the functionality. Regardless, it runs fine on my setup, tested on LPC55S69 (using #1676).

[kkitayam]

@tswan-quasi
Thank you for your helpful comments. I feel that your comments helped improve this PR.

[hathach]

superb! Thank you very much for yet another great PR. And thank @tswan-quasi for helping with reviewing. To be honest, I am still not catching up with UVC