Feature: stateful decoder interface

[Gnurou]

The only decoder interface that we support at the moment is the stateless one. For ease of use, clients should rather use a stateful API, close to the one proposed by Webcodecs.

This means:

• Defining a StatefulDecoder trait,
• Having an implementation of this trait that can wrap a stateless decoder and expose the stateful interface on top of it,
• When we have basic V4L2 support, writing another implementation of this trait to support stateful V4L2 devices.