Base class to define the interface of a custom command parser.

In order to facilitate the support of common custom commands by controller plugins, a custom command parser object can be used. A set of custom command parsers are created by the controller plugin (one for each interface supported), and added to the custom command parser manager in the controller framework. The controller plugin must also derive from a set of concrete interfaces (provided in the form of mixin classes). Then, a custom command supported by one of the custom command parsers will appear to the controller as a concrete API call, rather than a call to its own custom command API.