Generate custom command gateway interface

Instead of using the `CommandGateway` directly, we should generate a type safe, custom command interface and feed this to the `CommandGatewayFactory`.

See:

```kotlin
@Bean
  fun myCommandGateway(commandBus: CommandBus): MyCommandGateway = CommandGatewayFactory.builder()
    .commandBus(commandBus).build().createGateway(MyCommandGateway::class.java)
```