Query: Introduce client projections in SelectExpression

- ClientProjections is alternate when ProjectionMember binding is not possible using indexes
- ClientProjections is separate from SelectExpression.Projection where former holds complex client side mapped projections and later holds only scalars
- What it enables
  - Single result subquery is not joined right away
  - pendingCollections are removed
  - Ability to bind with above subquery translations after projection has converted to index based binding (enabling pending selector future change)
  - Ability to bind client projections smoothly like ProjectionMember binding so suppose translations post-client eval where it is supported
  - Grouping.FirstOrDefault can add subquery projection which applies separately so we can combine aggregate/non-aggregate projection on grouping
- Introduce CollectionResultExpression which holds info on how to create collection for a subquery
- ApplyProjection converts projectionMember/Index based bindings to actual scalar projection and updates shaper in same pass
- Unique collectionId are assigned by shaper
- Change in flow to projection to let sqlTranslator determines translatibility before processing for client side
- Regression in collection over single result subquery
  - Earlier we applied single result right away copying over pending collection to outer which allowed single result subquery to convert apply to join
  - Now all client projections apply at same time so pending collection (equivalent subquery) is applied inside single result subquery so we fail to convert apply to join

Ground work for #20291, #12088, #13805
Resolves #23571

src/EFCore.Relational/Query/CollectionResultExpression.cs

@@ -0,0 +1,99 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Utilities;
+
+namespace Microsoft.EntityFrameworkCore.Query
+{
+    /// <summary>
+    ///     <para>
+    ///         An expression that represents creation of a collection in <see cref="ShapedQueryExpression.ShaperExpression" /> for relational providers.
+    ///     </para>
+    ///     <para>
+    ///         This type is typically used by database providers (and other extensions). It is generally
+    ///         not used in application code.
+    ///     </para>
+    /// </summary>
+    public class CollectionResultExpression : Expression, IPrintableExpression
+    {
+        /// <summary>
+        ///     Creates a new instance of the <see cref="CollectionResultExpression" /> class.
+        /// </summary>
+        /// <param name="projectionBindingExpression"> An expression reprensenting how to get the subquery from SelectExpression to get the elements. </param>
+        /// <param name="navigation"> A navigation associated with this collection, if any. </param>
+        /// <param name="elementType"> The clr type of individual elements in the collection. </param>
+        public CollectionResultExpression(
+            ProjectionBindingExpression projectionBindingExpression,
+            INavigationBase? navigation,
+            Type elementType)
+        {
+            ProjectionBindingExpression = projectionBindingExpression;
+            Navigation = navigation;
+            ElementType = elementType;
+        }
+
+        /// <summary>
+        ///     The expression to get the subquery for this collection.
+        /// </summary>
+        public virtual ProjectionBindingExpression ProjectionBindingExpression { get; }
+        /// <summary>
+        ///     The navigation if associated with the collection.
+        /// </summary>
+        public virtual INavigationBase? Navigation { get; }
+        /// <summary>
+        ///     The clr type of elements of the collection.
+        /// </summary>
+        public virtual Type ElementType { get; }
+
+        /// <inheritdoc />
+        public override Type Type => ProjectionBindingExpression.Type;
+        /// <inheritdoc />
+        public override ExpressionType NodeType => ExpressionType.Extension;
+
+        /// <inheritdoc />
+        protected override Expression VisitChildren(ExpressionVisitor visitor)
+        {
+            Check.NotNull(visitor, nameof(visitor));
+
+            var projectionBindingExpression = (ProjectionBindingExpression)visitor.Visit(ProjectionBindingExpression);
+
+            return Update(projectionBindingExpression);
+        }
+
+        /// <summary>
+        ///     Creates a new expression that is like this one, but using the supplied children. If all of the children are the same, it will
+        ///     return this expression.
+        /// </summary>
+        /// <param name="projectionBindingExpression"> The <see cref="ProjectionBindingExpression" /> property of the result. </param>
+        /// <returns> This expression if no children changed, or an expression with the updated children. </returns>
+        public virtual CollectionResultExpression Update(ProjectionBindingExpression projectionBindingExpression)
+        {
+            Check.NotNull(projectionBindingExpression, nameof(projectionBindingExpression));
+
+            return projectionBindingExpression != ProjectionBindingExpression
+                ? new CollectionResultExpression(projectionBindingExpression, Navigation, ElementType)
+                : this;
+        }
+
+        /// <inheritdoc />
+        public virtual void Print(ExpressionPrinter expressionPrinter)
+        {
+            expressionPrinter.AppendLine("CollectionResultExpression:");
+            using (expressionPrinter.Indent())
+            {
+                expressionPrinter.Append("ProjectionBindingExpression:");
+                expressionPrinter.Visit(ProjectionBindingExpression);
+                expressionPrinter.AppendLine();
+                if (Navigation != null)
+                {
+                    expressionPrinter.Append("Navigation:").AppendLine(Navigation.ToString()!);
+                }
+                expressionPrinter.Append("ElementType:").AppendLine(ElementType.ShortDisplayName());
+            }
+        }
+    }
+}

src/EFCore.Relational/Query/EntityProjectionExpression.cs

@@ -216,5 +216,8 @@ public virtual void AddNavigationBinding(INavigation navigation, EntityShaperExp
                 ? expression
                 : null;
         }
+
+        /// <inheritdoc />
+        public override string ToString() => $"EntityProjectionExpression: {EntityType.ShortName()}";
     }
 }