V16: Siblings endpoints (#19657)

* PoC implementation

* Move to controller base

* Implement solution that seems worse, but works better

* Don't require parent key in repository method

* Fix typos

* Add siblings for data type, media type and media

* Add endpoint for template

* Add DocumentType and DocumentBlueprint controllers

* Fix naming

* Fix case if siblings are under root

* Take item ordering into account

not all entities are ordered by sort order

* Add default implementation

* Fix parentkey

* Add tests

* Format optimizations for split view

* Add test covered requirement to description

* Cover positive case and make test case output more readable

* reduce allocations

* Clarify test

---------

Co-authored-by: Migaroez <geusens@gmail.com>

src/Umbraco.Infrastructure/Persistence/NPocoSqlExtensions.cs

@@ -1298,6 +1298,14 @@ namespace Umbraco.Extensions
 
         #region Utilities
 
+        public static Sql<ISqlContext> AppendSubQuery(this Sql<ISqlContext> sql, Sql<ISqlContext> subQuery, string alias)
+        {
+            // Append the subquery as a derived table with an alias
+            sql.Append("(").Append(subQuery.SQL, subQuery.Arguments).Append($") AS {alias}");
+
+            return sql;
+        }
+
         private static string[] GetColumns<TDto>(this Sql<ISqlContext> sql, string? tableAlias = null, string? referenceName = null, Expression<Func<TDto, object?>>[]? columnExpressions = null, bool withAlias = true, bool forInsert = false)
         {
             PocoData? pd = sql.SqlContext.PocoDataFactory.ForType(typeof (TDto));