Make NullValue and Setting public (elastic#2496)

Author: swallez

specification/_spec_utils/utils.ts

@@ -24,12 +24,12 @@ import { Stringified } from '@spec_utils/Stringified'
  * to a missing value. It is used for exemple in settings, where using the `NullValue` for a setting will reset
  * it to its default value.
  */
-type NullValue = null
+export type NullValue = null
 
 /**
  * Settings in Elasticsearch are values that can be reset to their default by setting them to the `null` value.
  *
  * @es_quirk Because of how they are implemented internally, settings are always returned as strings, even
  *           if their value has been set using a primitive type.
  */
-type Setting<T> = Stringified<T> | NullValue
+export type Setting<T> = Stringified<T> | NullValue