Add spring-boot-http-codec as spring-boot-webclient needs it

108ca649 trimmed down the dependencies of spring-boot-starter-test
but it went too far. Marking spring-boot-webclient as non-transitive
meant that spring-boot-http-codec was no longer on the classpath.
However, it's a required dependency of the spring-boot-webclient
auto-configuration due to an auto-configuration after reference to
CodecsAutoConfiguration.class.

This commit adds spring-boot-http-codec as a direct non-transitive
dependency. This should be sufficient to allow
spring-boot-webclient's auto-configuration to function while still
minimizing the effects of adding spring-boot-starter-test to an app.

See gh-4629

starter/spring-boot-starter-test/build.gradle

@@ -25,6 +25,9 @@ dependencies {
 	api(project(":module:spring-boot-http-client")) {
 		transitive = false
 	}
+	api(project(":module:spring-boot-http-codec")) {
+		transitive = false
+	}
 	api(project(":module:spring-boot-restclient")) {
 		transitive = false
 	}