This adds a base class for SDCP devices. Not all functionality has been
fully tested, in particular the code to verify the model certificate is
most likely broken or incomplete. One problem there is that there is no
code to find the root CA to trust.
See: #257
It can be convenient for device users to check what it supports, without
having multiple functions to check each single feature.
So expose this and add tests.
Allows drivers to define more fine grained features for devices, not
strictly depending on assumptions we can make depending on the
implemented vfuncs.
We keep this per class but could be in theory moved to each instance.
In any case, added an utility function to initialize it in the way we
can ensure that we've a consistent way for setting them across all the
devices.
This is primarily useful for SPI devices. These devices sometimes needs
a combination of an SPI and HID device, so discovery is a bit more
complicated.
