Support lazy loading preinstalled Snaps

[Mrtenz]

Currently all preinstalled Snaps must be provided to the SnapController through the constructor, meaning all preinstalled Snaps must be fetched at app initialisation time. This may become problematic when there are many preinstalled Snaps, resulting in a long initialisation time.

We should consider allowing lazy loading of preinstalled Snaps. Instead of providing the full JSON, we can provide metadata for how the Snap should be fetched, and lazily fetch when the Snap is actually invoked for the first time (e.g., in SnapController.startSnap).