Feature: device discovery

[Gnurou]

Right now using cros-codecs requires to know which decoding hardware is available and how to access it. More generic software however would benefit from the ability to discover the accelerated decoding capabilities of the system.

The idea would be to get a list of devices, which in turn can be queried about their capabilities (supported codecs, limits, etc), and finally instantiated into a decoder or encoder instance.

This may require the use of trait objects to abstract the backend used, or maybe we can devise a backend-aware interface that would be a bit more work for the client...