Introduce PartEvent

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

spring-web/src/test/resources/org/springframework/http/codec/multipart/no-body.multipart

@@ -0,0 +1,9 @@
+--boundary
+Part: 1
+
+
+--boundary
+Part: 2
+
+a
+--boundary--