Models: Create definitions for everything in AaiComponents

[EricWittmann]

No description provided.

[lbroudoux]

Maybe this one has been addressed through #187 ?

[EricWittmann]

No the issue here is that we're re-using the same models for definitions and non-definitions (in-line usage). So these for example:

https://github.com/Apicurio/apicurio-data-models/blob/master/src/main/java/io/apicurio/datamodels/asyncapi/models/AaiComponents.java#L44-L47

This results in fewer models but more difficult visitation and analysis. Because when you visit e.g. an AaiMessage you don't know if it's inline or a definition found in the /components. The result is code like this:

https://github.com/Apicurio/apicurio-data-models/blob/master/src/main/java/io/apicurio/datamodels/asyncapi/io/AaiDataModelWriter.java#L309-L331

Where we try to figure out from the parent what kind of thing we're writing out. If instead we had a AaiMessageDefinition subclass with its own visitMessageDefinition, we would just know. It makes some other things slightly more difficult (e.g. if you have a visitor that needs to analyze Messages and MessageDefinitions then you need to implement the visit method for both of those). But it's how we do things in the OpenAPI model, so if nothing else we should be consistent.