Support repeatable writes in FormHttpMessageConverter#writeMultipart

[NicklasWallgren]

The FormHttpMessageConverter#writeMultipart method should support repeatable bodies, rather than only supporting single-write. Today StreamingHttpOutputMessage.Body#repeatable returns false, by default.

FormHttpMessageConverter#writeForm got support for repeatable writes in
6dd93d4

Continuation of #31449 (comment)

[rstoyanchev]

Some parts may not be repeatable. To find out for sure, it is necessary to iterate parts upfront, find a compatible message converter for each part, and check with the converter if the given (part) object is repeatable. If all parts are repeatable, then the multipart write is repeatable.