[I] Python SDK: add store_consumer_offset and get_consumer_offset [iggy]

["ethanlin01x (via GitHub)" <gi...@apache.org>]

ethanlin01x opened a new issue, #3997:
URL: https://github.com/apache/iggy/issues/3997

   ### Description
   
   The Python client cannot read or write a consumer offset directly. `poll_messages` can store an offset through `auto_commit=True`, and `IggyConsumer` has `store_offset()` / `delete_offset()`, but neither lets a caller commit an offset it chooses or ask the server what is currently stored. Every other SDK exposes the `ConsumerOffsetClient` methods on the client itself.
   
   This blocks the common pattern of polling with `auto_commit=False` and committing only after the application has finished processing a batch. It also makes offset state impossible to inspect from Python, for example when checking consumer lag.
   
   ### Affected area / component
   
   Python SDK
   
   ### Proposed solution
   
   Add two methods to `IggyClient` in `foreign/python/src/client.rs`, mapping to the `ConsumerOffsetClient` trait:
   
   ```python
   await client.store_consumer_offset(
       stream="s", topic="t", consumer=Consumer.Single("my-app"), offset=42, partition_id=0
   )
   
   info = await client.get_consumer_offset(
       stream="s", topic="t", consumer=Consumer.Single("my-app"), partition_id=0
   )
   if info is not None:
       print(info.partition_id, info.current_offset, info.stored_offset)
   ```
   
   `partition_id` stays optional, like on `poll_messages`, so a consumer group can resolve the member's assigned partition on the server.
   
   `get_consumer_offset` needs a new `ConsumerOffsetInfo` pyclass with `partition_id`, `current_offset` and `stored_offset` getters. It returns `None` when the server has no offset for that consumer, matching `get_consumer_group`.
   
   
   
   ### Alternatives considered
   
   _No response_
   
   ### Contribution
   
   - [x] I'm willing to submit a pull request to implement this feature
   
   ### Good first issue
   
   - [ ] I think this could be a good first issue for a new contributor


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