Fix erroneous encoding via contextual serialization

[n0900]

Issue:
In bigger structures, when the specific class was known, kotlinx.serialization would pick the generic polymorphic serializer for InputDescriptor instead of the specific InputDescriptorSerializer which is defined for the interface. In this case the type discriminator was added.

Solution:
To address this we force the serializer to use the contextual serializer instead which we set to the appropriate InputDescriptorSerializer.
To this end we also add a BaseInputDescriptorSerializer which only calls the DifInputDescriptor serializer without adding the type discriminator.
We need contextual serialization because InputDescriptor as an interface allows for open polymorphism and in our use case specifically QesInputDescriptor is not defined in the DIF artifact which means the Serializer cannot be hard-coded into the PresentationDefintion class.

Also added some tests.

[nodh]

nice!