Entity Data Picker: Data editor and value converter (#20661)

* change property value to an object

* add const for picker data source type

* Add value editor and converter server-side

* register schema for property editor + move settings ui

---------

Co-authored-by: kjac <kja@umbraco.dk>
Co-authored-by: Niels Lyngsø <nsl@umbraco.dk>

src/Umbraco.Core/Models/EntityDataPickerValue.cs

@@ -0,0 +1,8 @@
+namespace Umbraco.Cms.Core.Models;
+
+public sealed class EntityDataPickerValue
+{
+    public required IEnumerable<string> Ids { get; set; }
+
+    public required string DataSource { get; set; }
+}