Introduce MultipartHttpMessageConverter #36255
Description
As part of #33263, we would like to parse multipart messages without relying on the Servlet container support. This means we cannot depend on high-level Servlet container artifacts. We could consider depending on some HTTP client support, or a third party library. We initially did so for our reactive stack in #19114 but we later had to implement our own multipart parser to solve many problems.
After considering our options, we have decided to introduce a dedicated MultipartHttpMessageConverter that will support both reading and writing multipart messages. We will port our existing reactive MultipartParser implementation to an imperative model. This also means that equivalent Part, FilePart and FormFieldPart dedicated types will be added.