Annotate optional inputs with @Optional

Using the plugin with Spring Integration resulted in a failure:

````
* What went wrong:
Some problems were found with the configuration of task ':spring-integration-amqp:springioAlternativeDependenciesCheck'.
> No value has been specified for property 'configurations'.
> No value has been specified for property 'alternatives'.
````

This PR addresses the problem by annotating both configurations and alternatives as `@Optional`