This commit adds a new convenience ResponseCreator implementation that delegates to a ClientHttpRequestFactory to perform a request and use the response.

For further convenience, a factory method based on a RestTemplate is also added: MockRestResponseCreators#byExecutingRequestUsing. This acts as a pseudo-DSL for fluently setting the ResponseCreator in a ResponseActions.

The primary purpose of this implementation is to perform an actual request to an external service and use the response in MockMVC.

Closes gh-26381

Comment From: simonbasle

@rstoyanchev I have reverted the previous implementation and introduced ExecutingResponseCreator instead per your suggestion.

Comment From: simonbasle

mmh I realize just now that the factory method in MockRestResponseCreators doesn't make much sense, because the ExecutingResponseCreator MUST be instantiated early on, before the RestTemplate's factory is changed...

I wonder how to improve the discoverability of the feature 🤔