Draft - Feature/equals hashcode in client generation

[denvitaharen]

This is an draft to discuss, its lacks test, the formatting is a bit off and can be made better, but its up for discussion.

I missed a generated equals and dashcode in my models, so I thought that I tried adding it. I think I got all cases checked, but before I add more time in this it should be discussed.

1. Do we want this? Other Openapi client generators generates this (so it breaks stuff if we change to this generator)
2. If we want it, should it always generate methods and not being able do disable? Or should you have to enable it? Or should it be enabled by default with possibility to disabled?

This is an example of how the generated method looks:

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Animal model = (Animal) o;

        return Objects.equals(born, model.born) &&
        Objects.equals(deceased, model.deceased);
    }

     @Override
     public int hashCode() {
         return Objects.hash(born,
         deceased);
     }

[denvitaharen]

Sorry, I accidentally pushed an branch with commits from my last PR, its only this commits that's 447bb46

I will try and fix the branch tomorrow.