Add body method and common interface to responses in ClientWithResponses #240

Description

Simplest use case - we need to check status code and in case of error - print the response body.
The generator generates:

type createObjectResponse struct {
	Body         []byte
	HTTPResponse *http.Response
	JSON201      *ObjectCreateResponse
}

// Status returns HTTPResponse.Status
func (r createObjectResponse) Status() string {
	if r.HTTPResponse != nil {
		return r.HTTPResponse.Status
	}
	return http.StatusText(0)
}

// StatusCode returns HTTPResponse.StatusCode
func (r createObjectResponse) StatusCode() int {
	if r.HTTPResponse != nil {
		return r.HTTPResponse.StatusCode
	}
	return 0
}

the HTTPResponse doesn't have body anymore, since it was consumed during parsing and populating Body and JSON201. Since every response is a different struct type - you would have to handle them case by case in every place you perform a call. There's no way we can introduce common handler methods for all responses.

1. The idea (although I believe has some implications..) is to add a new method for each response GetBody() []byte (Body() is reserved by the field).
   This way we can now introduce our own common interface for the responses without introducing any interfaces to oapi-codegen.

type XXX interface {
	StatusCode() int
}

2. Additionally, we could add the following snippet to client-with-responses.tmpl so the interface is available out of the box.

type Response interface {
    Status()     string
    StatusCode() int
    GetBody()    []byte
}

PS: Currently, we have overriden the client-with-responses.tmpl in our project to have both of the mentioned improvements. I am still interested in your thoughts whether you would want to put those improvements into the oapi-codegen.

I'm ready to open a PR for the improvements if needed.