Add ability to convert command requests into command object controller parameter

Depends on #4.

 * `Identity:FooBar` → `Surfnet\StepupMiddleware\Identity\Command\FooBarCommand`
 * `Identity:Ns.FooBar` → `Surfnet\StepupMiddleware\Identity\Command\Ns\FooBarCommand`

Merges into feature/event-sourcing master PR.
This would be discovered during development based on configuration (the annotation) that is wrogn, so `UnexpectedValueException` or maybe even `InvalidArgumentException` might be better here.