Support for Map body and contentType = application/x-www-form-urlencoded for client requests

[rwinch]

RestClient supports a body of type MultiMap<String, ?> and content type of application/x-www-form-urlencoded. However, the default setup does not support Map<String, ?>. It would be nice if RestClient (and WebClient) supported Map<String, ?> as well.

One could argue that it is trivial for users to use MultiMap<String, ?> instead of Map<String, ?>, but Spring should not put the burden on the user when it can easily be implemented by the framework.

[rstoyanchev]

FormHttpMessageConverter supports writing MultiValueMap and will pick that up by default, even without an explicitly specified content-type.

I think we could support Map provided the content-type is also explicitly set to "application/x-www-form-urlencoded". That should be unambiguous.

[bclozel]

I already have changes ready for #33263 and I think this issue could be part of it. Here, we probably want to make FormHttpMessageConverter a "smart http converter" to better consider generic type information. @rstoyanchev I can take care of this one, unless you had something else in mind?