Reject commands with the message Not Yet Implemented.
Remove explicit @ParamConverter annot, hard-code command namespace.

Remove the need for an explicit @ParamConverter annotation by creating
a marker interface for commands and hard-coding the namespace commands
live in. The CommandParamConverter can now detect supported controller
action arguments.
Inline $json in json_decode() call.
Rename $object to $data throughout CommandParamConverter.
Expand invalid command structure error messages.