[sdk-platform-java] showcase test: refactor and migrate client interceptors to TestClientInitializer

context: https://github.com/googleapis/sdk-platform-java/pull/2737#discussion_r1591547671

There are a couple of slightly different flavored client interceptors implemented for tests. e.g. [ITApiVersionHeaders.java](https://github.com/googleapis/sdk-platform-java/pull/2737/files/219c8fdb150a33a9f2e44c442f4e91a586000ead#diff-0c74baec42410162926db3ec4be8e8f2c04fd9d6cf214ef5bbf407e7e350f0b5) [ITAutoPopulatedFields.java](showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITAutoPopulatedFields.java) [ITDynamicRoutingHeaders.java](showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITDynamicRoutingHeaders.java). We should try to migrate them into TestClientInitializer.