Add topic/partition assignment data to consumer group members

The `ConsumerAssignmentData` client model already provides `topic_name` and `partition_id` per consumer, but neither the `Consumer` model nor any loader utility currently fetches or stores this data.

https://github.com/confluentinc/vscode/blob/5fcff5b3e9be853615097248ed2c61eda6219f39/src/clients/kafkaRest/models/ConsumerAssignmentData.ts#L36-L91

Will need:
- A loader function to fetch assignments per consumer (similar to `fetchConsumerGroupMembers` in https://github.com/confluentinc/vscode/issues/3229)
- An assignments property on the `Consumer` model (https://github.com/confluentinc/vscode/issues/3299)

We'll need to figure out _when_ to load this data, since it could be helpful when seeing the topics for a given Kafka cluster, but only loading it in certain scenarios (e.g. expanding a single `ConsumerGroup`) may require too much manual clicking by the user.