[server][controller][vpj] Add PubSubConsumerAdapter::subscribe with PubSubPosition parameter

[sushantmane]

Add PubSubConsumerAdapter::subscribe with PubSubPosition parameter

Introduce a new overload of subscribe in PubSubConsumerAdapter that accepts a
PubSubPosition. This change is aimed at supporting non-numeric offsets. This allows
consumers to specify positions such as EARLIEST, LATEST, or an implementation-specific
position type. Implementors must map this PubSubPosition to the appropriate offset in
the underlying pub-sub system.

Enhancements to PubSub Consumer Functionality:

• PubSubConsumerAdapter:
  Added a new subscribe method that uses PubSubPosition to determine the starting
  offset for consumption. This method is marked with the UnderDevelopment annotation.
  (PubSubConsumerAdapter.java)

• PubSubPosition Updates:
  Added EARLIEST and LATEST positions to the PubSubPosition interface.
  (PubSubPosition.java)

• ApacheKafkaConsumerAdapter Implementation:
  Implemented the new subscribe method to handle PubSubPosition.

New Annotations:

• UnderDevelopment:
  Added a new annotation to indicate that an API is under development and not yet stable.
  (UnderDevelopment.java)

• VisibleForTesting:
  Added a new annotation to indicate that a method, class, or field is more visible than
  necessary for testing purposes. (VisibleForTesting.java)

How was this PR tested?

UT and E2ETest

Does this PR introduce any user-facing changes?

• No. You can skip the rest of this section.
• Yes. Make sure to explain your proposed changes and call out the behavior change.

[sixpluszero]

Overall looks good. Thanks for the detailed PR description and help me to get understanding of the purpose. I left a few comments but overall it is in good shape to me.

[sushantmane]

Thanks, @sixpluszero!

[lusong64]

Good comments! I left a recommendation for keeping the reference equal in one place.