Provide TestRestTemplate for @WebIntegrationTests

Add a ContextCustomizerFactory to provide TestRestTemplate as a bean for
tests annotated with WebIntegrationTests. Additionally provide support
for automatically expanding URLs of the form `/example` to
`http://localhost:${local.server.port}/example`.

Fixes gh-5227

spring-boot-test/src/main/resources/META-INF/spring.factories

@@ -1,6 +1,7 @@
 # Spring Test ContextCustomizerFactories
 org.springframework.test.context.ContextCustomizerFactory=\
 org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizerFactory,\
+org.springframework.boot.test.context.web.WebIntegrationTestContextCustomizerFactory,\
 org.springframework.boot.test.mock.mockito.MockitoContextCustomizerFactory
 
 # Test Execution Listeners