Improve zeek-client's enum handling and add get-nodes command

[ckreibich]

This PR contains two main parts:

• It refactors the translation of Zeek's enums to/from Python enum.Enum objects. This is a bit tricky since Zeek renders enums into fully-scoped strings ("Foo.Bar.VAL_1") with no immediate equivalent to such scoping in Python. The PR adds a new class hierarchy branch for enums to make the rendering of enums to/from Broker more similar to that of other records.

• More importantly, it adds a get-nodes command to support the new corresponding functionality on the Zeek side.

For a deployed configuration this might look as follows:

$ zeek-client get-nodes | jq .
{
  "instance-1": {
    "controller-swinetrek": {
      "cluster_role": null,
      "mgmt_role": "CONTROLLER",
      "pid": 2137201,
      "state": "RUNNING"
    },
    "instance-1": {
      "cluster_role": null,
      "mgmt_role": "AGENT",
      "pid": 2137200,
      "state": "RUNNING"
    },
    "logger-01": {
      "cluster_role": "LOGGER",
      "mgmt_role": null,
      "pid": 2137288,
      "state": "RUNNING"
    },
    "manager": {
      "cluster_role": "MANAGER",
      "mgmt_role": null,
      "pid": 2137289,
      "state": "RUNNING"
    },
    "worker-01": {
      "cluster_role": "WORKER",
      "mgmt_role": null,
      "pid": 2137291,
      "state": "RUNNING"
    },
    "worker-02": {
      "cluster_role": "WORKER",
      "mgmt_role": null,
      "pid": 2137290,
      "state": "RUNNING"
    }
  }
}