Auto-casting of DBFields

[chillu]

Follow up from #3. At the moment you have to be quite explicit with field types and casting. Those are implementation specific (e.g. Algolia wants dates as a timestamp, Elastic as RFC3339).

SilverStripe\SearchService\Service\IndexConfiguration:
  indexes:
    ingo:
      includeClasses:
        Page:
          fields:
            created:
              property: Created
              options:
                type: date

I think this should be handled transparently in IndexingInterface->configure() (for schema definitions) and IndexingInterface->addDocument() (for indexing). Or rather, DataObjectDocument->toArray() for casting, since the indexing interface is agnostic of DataObject/ORM concerns.

Types should stay configurable like in the example above, and the casting (e.g. to timestamp or RFC date) should be based on this.