Support mapping to fields

Issues #4357, #4855

This change adds fluent and lower-level APIs to set backing fields or properties with no fields as well as conventions for using the fields and a mechanism to provide control over when the property is used and when the field is used.

Also see #6266, #6267, and #6268.

src/Microsoft.EntityFrameworkCore.Relational/Metadata/Conventions/Internal/RelationalColumnAttributeConvention.cs

@@ -17,7 +17,8 @@ public class RelationalColumnAttributeConvention : PropertyAttributeConvention<C
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used 
         ///     directly from your code. This API may change or be removed in future releases.
         /// </summary>
-        public override InternalPropertyBuilder Apply(InternalPropertyBuilder propertyBuilder, ColumnAttribute attribute, PropertyInfo clrProperty)
+        public override InternalPropertyBuilder Apply(
+            InternalPropertyBuilder propertyBuilder, ColumnAttribute attribute, MemberInfo clrMember)
         {
             if (!string.IsNullOrWhiteSpace(attribute.Name))
             {

src/Microsoft.EntityFrameworkCore.Specification.Tests/DataAnnotationTestBase.cs

@@ -12,7 +12,6 @@
 using Microsoft.EntityFrameworkCore.Internal;
 using Microsoft.EntityFrameworkCore.Metadata;
 using Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal;
-using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using Microsoft.EntityFrameworkCore.Storage;
 using Xunit;
 
@@ -190,6 +189,29 @@ protected class PrivateMemberAnnotationClass
             private string PersonFirstName { get; set; }
         }
 
+        [Fact]
+        public virtual ModelBuilder Field_annotations_are_enabled()
+        {
+            var modelBuilder = CreateModelBuilder();
+
+            modelBuilder.Entity<FieldAnnotationClass>().Property<string>("_personFirstName");
+
+            Validate(modelBuilder);
+
+            Assert.True(GetProperty<FieldAnnotationClass>(modelBuilder, "_personFirstName").IsPrimaryKey());
+
+            return modelBuilder;
+        }
+
+        protected class FieldAnnotationClass
+        {
+#pragma warning disable 169
+            [Key]
+            [Column("dsdsd", Order = 1, TypeName = "nvarchar(128)")]
+            private string _personFirstName;
+#pragma warning restore 169
+        }
+
         [Fact]
         public virtual void NotMapped_should_propagate_down_inheritance_hierarchy()
         {