Support for linear SIMID ad without MediaFile

[marcusclip]

Per specs it doesn't seem possible to have a linear SIMID ad without a MediaFile.

However, there are some use cases where there simply isn't a simple video file that goes with the ad.

For example what if the SIMID ad plays the ad video in just a portion of the available area next to interactive content. This is a common format.

Another example is if the "video" is in fact an animation running on a canvas element.

The workaround would be to use a dummy black frame video, but I think it should be allowed to have a linear SIMID ad without a MediaFile.

Thanks!