Make serializing to multiple content-type easier

All my endpoint support serializing to JSON and CBOR. It is done by overriding the _prepare_response_content like that:

def _prepare_response_content(data):
        """ Serialize to JSON or CBOR"""
        response_mimetype = request.accept_mimetypes.best_match(
            [ MT_JSON, MT_CBOR ],
            default=MT_JSON
        )

        if response_mimetype == MT_JSON:
            content = json.dumps(data)
            headers = {"Content-Type" : "application/json" }

        if response_mimetype == MT_CBOR:
            content = cbor2.dumps(data)
            print(content.hex())
            headers = {"Content-Type" : "application/cbor" }

        return (content,headers )

It should work except that the function is inside a call to flask.jsonify() wich doesn't allow for anything but json serailization. So I have to override the entire response function to rewrite just that one line.
https://github.com/marshmallow-code/flask-smorest/blob/5ac7296f98e9829059c4a7835c1ca2ffc1fdbdf8/flask_smorest/response.py#L110-L111

If we replace flask.jsonify()by flask.make_response() it allow for much greater customization, including setting custom headers. That would break the default serialization but that can be fix by doing the equivalent of jsonify inside of _prepare_response_content:

def _prepare_response_content(data):
        content = json.dumps(data)
        headers = {"Content-Type" : "application/json" }
        return (content,headers )

I think this is a small modificatrion that would really make supporting multiple content-type much easier.

Is there any other impact that I have not forseen ? If this modification is acceptable I can do the merge request / documentation.

Start in flask_smorest/response.py at the referenced lines where flask.jsonify() is used. Check how replacing it with flask.make_response() would affect the existing JSON response behavior and custom headers, then verify that the default serialization remains intact; the issue does not name a test file.