Add support for openapi-contract-first development

[zhfeng]

No description provided.

[jamesnetherton]

For the templates used to generate the model sources, how likely is it that we'll need to update them frequently?

I guess it'd just be a case of copying them from the original Maven plugin source?

[zhfeng]

I think it could be not changed very offen. I only copy some templates which are related to the models generation. They are copied from io.swagger.codegen.v3:swagger-codegen-generators. And the only change is to add @RegisterForReflection in the generatedAnnotation.mustache with

@RegisterForReflection{{#serializableModel}}(serialization = true){{/serializableModel}}

[jamesnetherton]

We should probably drop a section into the docs about the codegen (as a follow up PR if you prefer), just so that it's clear for folks that they don't have to use the swagger maven plugins.