static Multipart of(String boundary, Publisher<? extends BodyPart> parts) {
        requireNonNull(parts, "parts");
        return new DefaultMultipart(boundary, StreamMessage.of(parts));
    }
Returns a new {@link Multipart} with the specified {@link BodyPart}s.
core/src/main/java/com/linecorp/armeria/common/multipart/Multipart.java