HTTP API - Use image for motion alter from HTTP post 
Hey, thanks for all the work on one of the best HB plugins. 

I recently have been experimenting with running my synology cameras through deepstack to get "AI" based alerts. [synoAI](https://github.com/djdd87/SynoAI) is a great repo for this if you use synology surveillance.

I can have it webhook back when it confirms AI based motion to trigger motion alert on ffmepg camera which is awesome. My problem arrises though that this is usually about 4-5 seconds from the original motion that tirggered my whole AI pipeline and by the time homebridge-camera-ffmpeg re requests a still image from the camera, the person or car has often already gone out of frame. 

The synoAI supports posting with the processed image which would be perfect if I could use it as the still image used by homebridge-camera-ffmpeg in the motion alert. 

Similarly if https://github.com/Sunoo/homebridge-smtp-motion could use the attached image from the email as the source of the still image, that would also solve this since SynoAI supports email notification. 

I have been considering other-ways I might be able to shuttle the image to homebridge and use a local path as the still image source as described here https://github.com/Sunoo/homebridge-camera-ffmpeg/issues/84#issuecomment-301576750


Cheers
