Add utility to convert between TopicPartition and PartitionInfo

A common usecase when working with manual assignments is to fetch information about the topics and partitions, and then do your assignment.
The interface for fetching the metadata returns a List<PartitionInfo>, but assign requires a Collection<TopicPartition>.

Converting between this classes is easy, but a bit verbose. Adding a simple conversion to a utility class, or on the classes themselves, would improve the API and user experience.