testing: explore more dedicated apps for integration tests

We currently have some sample apps in Envoy Mobile that perform actual requests to the internet, invoke very basic filter chains, and log the responses.

As Envoy Mobile has grown in complexity and capabilities, it has become clear that there is opportunity to improve this particular test coverage by adding more example apps that cover more complex integration test cases. For example, we could have apps that:
- Perform gRPC streaming requests and do some behavior with the results
- Construct and utilize complex filter chains (such as those that mutate contents, buffer, start and stop iteration, etc.)