Add Admin.describeCluster

[Nevon]

Returns metadata for all brokers in the cluster.

The API is roughly based on the Java Admin.describeCluster API. The big differences are:

1. I'm exposing the nodes under a brokers property instead of nodes. The id is still called a nodeId, but I don't think we really use the Node terminology anywhere, so I went with brokers instead.
2. The Java API has something called AuthorizedOperations that can be optionally included. I didn't add it as the current use-case doesn't need it, and I haven't actually looked into what it is or why you'd want it.

Usage is simply: await admin.describeCluster() and returns:

{
  brokers: [{ nodeId: 0, host: 'localhost', port: 9092 }],
  controller: 0,
  clusterId: 'foobar'
}

Fixes #647

[JaapRood]

@Nevon overall, seems like a straightforward PR worth adding. The Java API having exposed it is generally a good indication there's a good reason for it, #647 gives useful context as well. The two design decisions seems totally on point to me.

[JaapRood]

Looks great to me!

[JaapRood]

Short and sweet way to prevent a whole bunch of confusion, nice!