Clarify use of MultipartBodyBuilder with RestTemplate

[bclozel]

As seen in spring-projects/spring-boot#12579 and as shown in the reference documentation, developers are using MultipartBodyBuilder with RestTemplate in a pure Spring MVC application. In that context, the reactive-streams dependency bringing Publisher is not on classpath by default.

It seems that the MultipartBodyBuilder implementation requires that class to be on classpath.

We should make that dependency optional or avoid documenting this option with RestTemplate.

[rstoyanchev]

The original idea in #20682 was for MultipartBodyBuilder to be useful with both WebClient and RestTemplate but it came from usage of WebTestClient in #20666 and arguably is more idiomatic on the WebClient side. Over time we also added the asyncPart methods that do not apply to the RestTemplate so we might as well make this official and update the docs.