core: service: add simulated camera based on SITL

Could be kept super simple and low res, but would be really cool and useful to have a camera stream to go along with the onboard SITL option.

Would mean new users could do some basic piloting practice before using a real vehicle, and would also make it easier to do testing. Would be extra awesome if there was some way of specifying a simulated current to fight against, but I expect that's best handled in a web interface (either as settings for the SITL flight controller, or as an overlay to a web GCS).

The whole simulated camera would also likely be easier in a web GCS, because it could send geometry instead of pixels to the display, which are then rendered by the browser...