Refactor binding-model-api

mdsal-binding-model-api is overly generic. It originally supported two code generators, but one of them is gone and will never return.

mdsal-binding-java-api-generator makes a ton of assumptions and reverse engineering of the generated model to distill information from the generic class model. We are searching for Foo in 'implements Augmentation<Foo>' and 'implements Identifier<Foo>'.

This stems from the baseline objects known to mdsal-binding-model-api is Type (i.e. a Java type reference), GeneratedType (i.e. something we need to generate), GeneratedTransferObject (when its a class with properties), spiced up with isTypedef()/isUnion() property.

Considering that mdsal-binding-model-api is an interface between YANG and its projection to Java, the coupling between mdsal-binding-generator and mdsal-binding-java-api-generator and the binding specification needs to be firmed up.

This means making sure there is a yang-binding projection into GeneratedType class hierarchy, such that GeneratedType permits only its specializations – for example BuilderType, OpaqueType, ValueType, etc.

Generator dispatch in java-api-generator then should use instanceof checks coupled with GeneratedType being sealed in an if/else cascade to select the appropriate template.