static RestClientBuilder builder(String protocol, EndpointGroup endpointGroup) {
        return builder(SessionProtocol.of(requireNonNull(protocol, "protocol")), endpointGroup);
    }
Returns a new {@link RestClientBuilder} created with the specified {@code protocol}
and base {@link EndpointGroup}.

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