[Sling Models] Allow for extensible @Via providers

Currently, @Via support in Sling Models is limited to JavaBean properties. It would be useful to be able to extend this and allow for downstream projects to add new @Via providers.

Proposing to support this by extending the @Via annotation

@Via(value = "jcr:content", type = ChildResource.class)

The default type is BeanProperty (the current behavior).

New providers can be added by implementing a ViaProvider SPI and provide a marker class for use in the annotation.