Improve document schedule (#17535)

* Expose schedule date for on document get endpoint

* typo fix

* stupid stuff

* Enable content scheduling features in the publishing service

* Replace obsoleted non async calls

* Add content scheduling test

* Publush and schedule combination test

* More invariantCulture notation allignment and more tests

* Link up api with updated document scheduling

* More invariant culture notation allignment

* Fix breaking change

* Return expected status codes.

* Fix constructor

* Forward Default implementation to actual core implementation

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

* Forward default implementation to core implementation

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

* Make content with scheduling retrieval scope safe

---------

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Core/Constants-System.cs

@@ -89,5 +89,7 @@ public static partial class Constants
         /// The DataDirectory placeholder.
         /// </summary>
         public const string DataDirectoryPlaceholder = "|DataDirectory|";
+
+        public const string InvariantCulture = "*";
     }
 }