The subscribe method should return the consumer instead of nil, if consumer is already connected

**Is your feature request related to a problem? Please describe.**
I have a situation in which the client can request to subscribe again over the same topic and subscription name (due to unahandled connection disconnect). For such situations, the subscribe method should return the consumer instead of nil, since there is no other way to retreive an already subscribed consumer.

**Describe the solution you'd like**
The subscribe method should return the consumer instead of nil, if consumer is already connected. Or even better, there should be a mechanism to retrieve consumer by subscription name and topic.

**Describe alternatives you've considered**
I don't see any alternatives

**Additional context**
None

Start by tracing the Go client's Subscribe method and how it handles an existing consumer for the same topic and subscription name. Review the surrounding consumer lifecycle and existing tests, then define and verify whether repeated subscription returns the existing consumer or adds a retrieval mechanism.

Written by the indexing model from the issue text.