static BodyPart of(HttpHeaders headers, Publisher<? extends HttpData> publisher) {
        return builder().headers(headers).content(publisher).build();
    }
Returns a new {@link BodyPart} using the specified {@link HttpHeaders} and
{@link Publisher} of {@link HttpData}.
core/src/main/java/com/linecorp/armeria/common/multipart/BodyPart.java