Fetch consumer's committed offsets with metadata

**Describe the solution you'd like**

Currently `consumer.committed()` returns offset number. However Kafka stores optional metadata with committed offsets with metadata in struct `OffsetAndMetadata`, and the method `committed()` simply returns only numeric offset.

It should be possible to fetch committed offsets with metadata via aiokafka too without tinkering internals. Either additional API method should be added, or flag to existing method (returning different typed value).

**Additional context**

At least kafka-python (see [the implementation](https://github.com/dpkp/kafka-python/blob/master/kafka/consumer/group.py#L531-L569)) has additional flag in similar method to return committed offsets with metadata.