Add support for RestTestClient

[snicoll]

RestTestClient provides similar features as WebTestClient for imperative scenarios using RestClient. We should update our test configuration so that:

• It can be bound to MockMvc in a mocked environment
• It can be bound to a running server for integration tests on a running server.

[wilkinsona]

With reference to #47410, I think we may be missing a RestTestClient equivalent of SpringBootWebTestClientBuilderCustomizer so that any HttpMessageConverter customisation is applied to the auto-configured RestTestClient.

[donalmurtagh]

The documentation says that a RestTestClient instance for performing full end-to-end tests should be created via

var restTestClient = RestTestClient.bindToServer().baseUrl("http://localhost:8080").build();

If such an instance would not have any any HttpMessageConverter customisation applied to it, then the docs should be updated.

[wilkinsona]

That documentation is from Spring Framework where HTTP message converter customisation is something that has to be done manually on both the client- and server-side. While it doesn't mention message converters specifically, the general topic of customisation is covered in this section. If you think this could be improved, please open a Spring Framework issue.

[donalmurtagh]

I didn't notice that the link I posted was to Spring Framework docs, rather than Spring Boot docs. The point I was trying to make is that there ought to be somewhere in the Spring Boot docs that demonstrates how to create a RestTestClient with any HttpMessageConverter customisation applied to it.

For what it's worth, I managed to achieve this in my demo app (which only cares about JSON), like so.