Move RestTestClientBuilderCustomizer to spring-boot-test

Relocate `RestTestClientBuilderCustomizer` to `spring-boot-test`
and break the direct link to web-server by making use of
`spring.factories` and the new `BaseUrlProviders` class.

See gh-46356

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/spring-boot-applications.adoc

@@ -189,7 +189,7 @@ include-code::MyRandomPortWebTestClientTests[]
 
 TIP: javadoc:org.springframework.test.web.reactive.server.WebTestClient[] can also used with a xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.with-mock-environment[mock environment], removing the need for a running server, by annotating your test class with javadoc:org.springframework.boot.webflux.test.autoconfigure.AutoConfigureWebTestClient[format=annotation] from `spring-boot-webflux-test`.
 
-The `spring-boot-web-server-test` modules also provides a javadoc:org.springframework.boot.web.server.test.client.TestRestTemplate[] facility:
+The `spring-boot-retclient-test` modules also provides a javadoc:org.springframework.boot.restclient.test.TestRestTemplate[] facility:
 
 include-code::MyRandomPortTestRestTemplateTests[]
 

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/test-utilities.adoc

@@ -43,6 +43,8 @@ include-code::MyOutputCaptureTests[]
 == TestRestTemplate
 
 javadoc:org.springframework.boot.test.web.client.TestRestTemplate[] is a convenience alternative to Spring's javadoc:org.springframework.web.client.RestTemplate[] that is useful in integration tests.
+It's provided by the `spring-boot-restclient-test` module.
+
 You can get a vanilla template or one that sends Basic HTTP authentication (with a username and password).
 In either case, the template is fault tolerant.
 This means that it behaves in a test-friendly way by not throwing exceptions on 4xx and 5xx errors.