Use serializers for simple informational views like "metadata" views

[AlanCoding]

Some background in #276

With the latest changes (speculative) I still have the schema outputting this:

        "/service-index/": {
            "get": {
                "operationId": "service_index_retrieve",
                "description": "Link other resource registry endpoints",
                "tags": [
                    "service-index"
                ],
                "security": [
                    {
                        "cookieAuth": []
                    },
                    {
                        "basicAuth": []
                    },
                    {
                        "jwtAuth": []
                    },
                    {
                        "EDAJWTAuthentication": []
                    }
                ],
                "responses": {
                    "200": {
                        "description": "No response body"
                    }
                }
            }
        },
        "/service-index/metadata/": {
            "get": {
                "operationId": "service_index_metadata_retrieve",
                "tags": [
                    "service-index"
                ],
                "security": [
                    {
                        "cookieAuth": []
                    },
                    {
                        "basicAuth": []
                    },
                    {
                        "jwtAuth": []
                    },
                    {
                        "EDAJWTAuthentication": []
                    }
                ],
                "responses": {
                    "200": {
                        "description": "No response body"
                    }
                }
            }
        },

This isn't correct / complete, and it can't be unless the view uses a serializer. In these cases, the serializer would just be filled with read-only fields. It would do the same thing it does not, but using the serializer would allow OPTIONS to show the expected keys, and would allow OpenAPI spec to provide information about the format of the response.