Add modelerfour parameter to control whether Accept header parameter is generated

[daviwil]

Modelerfour has a flag called --modelerfour.always-create-content-type-parameter to control whether the Content-Type header parameter is added to operations automatically. We also automatically add the Accept header parameter automatically but we've been asked to put it behind a parameter so that the behavior can be disabled.

We should add a new parameter --modelerfour.always-create-accept-parameter and default it to true to retain the current behavior. This would require similar logic as we have for the Content-Type parameter so we can basically that same implementation.