Add Google Pub/Sub emulator support

I use Debezium Server and Google Pub/Sub but unfortunately, the current version doesn't support the Pub/Sub emulator.

This makes it hard to test locally and requires to have access to a Google Cloud project at all times. Supporting the emulator will also ease testing Debezium test cases in the CI.

Java and C# are the only languages that require an explicit implementation of the support, the rest depends on setting proper environment variables. Here's the relevant documentation for the implementation: https://cloud.google.com/pubsub/docs/emulator#pubsub-emulator-java