public static Retrofit of(String protocol, EndpointGroup endpointGroup) {
        return builder(protocol, endpointGroup).build();
    }
Returns a new {@link Retrofit} which sends requests to the specified {@link Endpoint} using
the specified {@code protocol}.

@throws IllegalArgumentException if the {@code protocol} is not one of the values in
                                 {@link SessionProtocol#httpValues()} or
                                 {@link SessionProtocol#httpsValues()}.
retrofit2/src/main/java/com/linecorp/armeria/client/retrofit2/ArmeriaRetrofit.java