Suggest rewrite `regexConsumer` with inheritance

I'm working on the batch receive feature #840. But before that, I think simplifying the implementation of `regexConsumer` and `multiTopicConsumer` is necessary.

In Java client, [`PatternMultiTopicsConsumerImpl`](https://github.com/apache/pulsar/blob/15a00137012669b0f6fd869ca5dbdf50898242ac/pulsar-client/src/main/java/org/apache/pulsar/client/impl/PatternMultiTopicsConsumerImpl.java) inherits from [`MultiTopicsConsumerImpl`](https://github.com/apache/pulsar/blob/2d6b534ef72824ffb5b2f76568ca8346bbc058c6/pulsar-client/src/main/java/org/apache/pulsar/client/impl/MultiTopicsConsumerImpl.java). But Go client implements [`multiTopicConsumer`](https://github.com/apache/pulsar-client-go/blob/5108332c9dd4cb454c26804304bffb82eeffc713/pulsar/consumer_multitopic.go#L33) and [`regexConsumer`](https://github.com/apache/pulsar-client-go/blob/5108332c9dd4cb454c26804304bffb82eeffc713/pulsar/consumer_regex.go#L39) separately for now.

I suggest rewriting [`regexConsumer`](https://github.com/apache/pulsar-client-go/blob/5108332c9dd4cb454c26804304bffb82eeffc713/pulsar/consumer_regex.go#L39) with inheritance just like Java client. If so, further feature development could be easier.

Feel free to correct me if my understanding is inaccurate. cc @wolfstudy @merlimat @nodece

Start by reading pulsar/consumer_regex.go and pulsar/consumer_multitopic.go, then compare the linked Java PatternMultiTopicsConsumerImpl and MultiTopicsConsumerImpl implementations. Determine the intended shared structure and confirm how it affects the batch receive work in issue #840. Done means the consumer implementations are simplified in the agreed design without breaking their existing behavior.

Written by the indexing model from the issue text.