public static ArmeriaRetrofitBuilder builder(String protocol, EndpointGroup endpointGroup) {
        return builder(SessionProtocol.of(requireNonNull(protocol, "protocol")), endpointGroup);
    }
Returns a new {@link ArmeriaRetrofitBuilder} that builds a client that sends requests to
the specified {@link EndpointGroup} 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