This commit applies similar changes already contributed to the
`HttpMessageConverter` stack for MVC applications.
Since there was no auto-configuration for Kotlinx JSON Serialization on
the reactive side, this commit adds a relevant `CodecCustomizer` that
will use an available `Json` bean and use it to configure a codec that:
* will only consider `@Serializable`-annotated types if another JSON
library is available
* will use a broader support with Kotlinx Serialization otherwise
Fixes gh-48070
Relocate `WebTestClientBuilderCustomizer` to `spring-boot-test`
and break the direct link to web-server and http-codec by making
use of `spring.factories` and the new `BaseUrlProviders` class.
A new `spring-boot-test-integration-test` module has also been
added to ensure hold the previous tests.
See gh-46356
Brian Clozel
Andy Wilkinson
Moritz Halbritter
Phillip Webb
Stéphane Nicoll