[Command] Allow enum and newtype

[Freyskeyd]

Command can already be use with enum and newtype but the Command derive only support struct.

#[derive(Debug)]
struct AppendItem(i64);

impl Command for AppendItem {
    type Event = ItemAppended;

    type Executor = ExampleAggregate;

    type ExecutorRegistry = AggregateInstanceRegistry<ExampleAggregate>;

    fn identifier(&self) -> String {
        "example_aggregate".to_string()
    }
}

It could be possible to have an enum which can be dispatch to the CommandExecutor.

Some questions need to be resolved first:

• How to deal with command's identity ?
• Is it really necessary ?