This commit introduces the PartEvent API. PartEvents are either
- FormPartEvents, representing a form field, or
- FilePartEvents, representing a file upload.
The PartEventHttpMessageReader is a HttpMessageReader that splits
multipart data into a stream of PartEvents. Form fields generate one
FormPartEvent; file uploads produce at least one FilePartEvent. The last
element that makes up a particular part will have isLast set to true.
The PartEventHttpMessageWriter is a HttpMessageWriter that writes a
Publisher<PartEvent> to a outgoing HTTP message. This writer is
particularly useful for relaying a multipart request on the server.
Closes gh-28006
be7fa3aaa8