Support JdkClientHttpConnector in DefaultWebTestClientBuilder

[mhalbritter]

In org.springframework.test.web.reactive.server.DefaultWebTestClientBuilder#initConnector there are checks for Reactor Netty, Jetty and Apache Http for the ClientHttpConnector implementation. As there is now a JdkClientHttpConnector which works without external dependencies, I think it would be nice if DefaultWebTestClientBuilder could fallback to that instead of throwing an exception.

I don't know if there are more places which do this auto-detection of ClientHttpConnector, but if there are, it would be nice to update them to include JdkClientHttpConnector.

[mhalbritter]

org.springframework.web.reactive.function.client.DefaultWebClientBuilder#initConnector has the same detection, and there JdkClientHttpConnector is already integrated. I will create a PR which updates DefaultWebTestClientBuilder.

[mhalbritter]

See #29645

[jhoeller]

Superseded by #29645.