Require handlers to return *Response directly

[diamondburned]

This commit changes the generated handler methods inside ServerInterface
to directly return a *Response value instead of requiring it to call
render.Render manually. This results in much cleaner handler code.

While this change breaks all existing code, it is not a significant
change by any means, so most changes can be regexed away to work.

As an example, petstore's FindPetByID is now

func (p *PetStore) FindPetByID(w http.ResponseWriter, r *http.Request, id int64) *Response {
	p.Lock.Lock()
	defer p.Lock.Unlock()

	pet, found := p.Pets[id]
	if !found {
		return FindPetByIDJSONDefaultResponse(Error{fmt.Sprintf(petNotFoundMsg, id)}).Status(http.StatusNotFound)
	}

	return FindPetByIDJSON200Response(pet)
}

compared to before

func (p *PetStore) FindPetByID(w http.ResponseWriter, r *http.Request, id int64) {
	p.Lock.Lock()
	defer p.Lock.Unlock()

	pet, found := p.Pets[id]
	if !found {
		render.Render(
			w, r,
			FindPetByIDJSONDefaultResponse(Error{fmt.Sprintf(petNotFoundMsg, id)}).Status(http.StatusNotFound),
		)
		return
	}

	render.Render(w, r, FindPetByIDJSON200Response(pet))
}

Ideally, the above piece of code would look something like

func (p *PetStore) FindPetByID(w http.ResponseWriter, r *http.Request, id int64) *Response {
	p.Lock.Lock()
	defer p.Lock.Unlock()

	pet, found := p.Pets[id]
	if !found {
		return FindPetByIDJSON404Response(Error{fmt.Sprintf(petNotFoundMsg, id)})
	}

	return FindPetByIDJSON200Response(pet)
}

where a FindPetByIDJSON404Response is properly generated by the OpenAPI schema having the 404 status code documented, but it isn't in the example schema for some reason.

[karitham]

lgtm, waiting on another review before merge since it's a big breaking change.

[zacharyburkett]

This is a great improvement to overall usage. Good stuff! 🔥