Add DescribeTopics and DescribeCluster support for admin client API

[alihan-synnada]

Closes #614

Adding DescribeTopics now and also potentially DescribeCluster in this PR as per #614

I'll write the docs and tests, and then mark as ready for review

[alihan-synnada]

Is it really worth adding a dependency to the uuid crate when Uuid is only used here? We might as well remove the dependency and only return a (high_bits, low_bits) tuple.

[alihan-synnada]

I added DescribeCluster support as well. DescribeConsumerGroups should also be relatively easier to implement since the some of the types it returns (Node and AclOperation) are implemented now.

[alihan-synnada]

I'm satisfied with the PR myself but there are a few points in my mind:

• I'm not sure if the tests are covering enough ground.
• I couldn't decide where to place structs and enums so they are somewhat all over the place.
• Should we add the uuid crate as a dependency? The Uuid type is more descriptive and the uuid crate is rather lean when no features are enabled. On the other hand, it's only used for topic_id in TopicDescription so we can remove the dependency and simply use (u64, u64) or u128 instead.

[alihan-synnada]

@davidblewett @benesch Any opportunities to review and possibly merge within next week?