Add more data references

Short version: Add entity_type_reference, [entity_type]_bundle_reference, config_item_reference data types, maybe more.

Long version:
I wonder why all world declares references in config items as strings and does all the option and validation logic in form land.
As a developer, i want to declare my config item like this and possibly get autogenerated widgets:

    entity_type:
      type: entity_type_reference
    bundles:
      type: sequence
      label: Bundles
      sequence:
        type: entity_type_reference
        entity_type: %parent.entity_type
        label: Bundle
    config_item:
      type: config_item_reference

and not like this:

    entity_type:
      type: string
    bundles:
      type: sequence
      label: Bundles
      sequence:
        type: string
        label: Bundle
    config_item:
      type: string

I stumbled across core's language_reference so guess this should be possible.

Is it possible, and can it go into this module?