Static case insensitive property check definitions

[chillu]

**Subtask of silverstripe/silverstripe-asset-admin#316 **

At the moment we have CaseInsensitiveFieldAccessor which infers field existence of ViewableData and allows us to reference a urlSegment property in GraphQL, but read/write from URLSegment in the SS ORM.

This is naturally a bit of magic, but its too opaque by placing it in the field resolution. We should move this to the GraphQL type definition instead. This allows us to make mapping choices more explicit.

Rough API sketch:

use Chillu\GraphQL\DataObjectTypeCreator;
use Chillu\GraphQL\LowercaseNameTransformer;
$type = (new DataObjectTypeCreator('SiteTree'))
  ->addFieldTransformer(new LowercaseNameTransformer())
  ->setFields([
    'ID' => [], // transformer sets ['name' => 'id'] automatically
    'URLSegmentField' => ['name' => 'urlSegmentField'] // manual mapping
  )
  ->toType();

TODO Figure out how to retain mapping for query resolvers

This is also required for the type scaffolding