Expose camera presets to web pages

[youennf]

Following on https://github.com/w3c/mediacapture-main/issues/739, current API is making it difficult for web developers to select constraints when they are tied with each other. Also, it makes it usually hard for web developers to select particular native presets for which they could expect the best performance as user agents would limit processing such as downsampling. One possibility would be to expose native camera presets to web developers so that they can generate their constraints given to applyConstraints more easily. A preset could be defined as a set of constraints (width, heigh, frame rate, pixel format...) with discrete values.