Admin client: list offsets per topic/partition

**Describe the solution you'd like**
A new function in the admin client that lets me retrieve the offsets (earliest/latest) of each partition.

**Additional context**
Confluent's admin client has [list_offsets](https://docs.confluent.io/platform/current/clients/confluent-kafka-python/html/_modules/confluent_kafka/admin.html#AdminClient.list_offsets).

My use case is retrieving the number of messages behind per consumer-group/topic. `list_consumer_group_offsets` provides the offsets per consumer group, and I need the offset of the latest messages.