[sdk-platform-java] Client libraries should be mockable for testing

**Is your feature request related to a problem? Please describe.**
I am attempting to write JUnit tests for my code that integrates with Dialogflow CX. However, the `SessionsClient` does not implement a service interface, and the business methods (particularly `detectIntent(DetectIntentRequest)`) are all `final`. This means that it is impossible to use a test double, either a JDK interface proxy or a CGLib subclass.

**Describe the solution you'd like**
The service APIs should implement a business service interface that can be mocked.

**Describe alternatives you've considered**
No alternatives considered; this is fundamental baseline functionality for an integration library.