Add RuntimeFeature.DefaultImplementationsOfInterfaces (#25943)

The flag indicates that the runtime supports default implementation of interfaces.

Since this is a prerelease feature that is not complete yet (and we don't want to accidentally ship it), the flag is keyed off a newly added `IsPrerelease` property. This property is expected to be flipped to `false` in release branches.

This corresponds to dotnet/coreclr#15358.

Author: MichalStrehovsky

dir.props

@@ -23,6 +23,11 @@
     <IsDotNetFrameworkProductAssembly>true</IsDotNetFrameworkProductAssembly>
   </PropertyGroup>
 
+  <!-- Indicates this is not an officially supported release. Release branches should set this to false. -->
+  <PropertyGroup>
+    <IsPrerelease>true</IsPrerelease>
+  </PropertyGroup>
+
   <!-- Common repo directories -->
   <PropertyGroup>
     <ProjectDir>$(MSBuildThisFileDirectory)</ProjectDir>

src/System.Runtime/ref/System.Runtime.cs

@@ -6828,6 +6828,9 @@ public enum LoadHint
     }
     public static class RuntimeFeature
     {
+#if FEATURE_DEFAULT_INTERFACES
+        public const string DefaultImplementationsOfInterfaces = nameof(DefaultImplementationsOfInterfaces); 
+#endif
         public static bool IsSupported(string feature) { throw null; }
     }
     [System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)]

src/System.Runtime/ref/System.Runtime.csproj

@@ -5,6 +5,10 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <IsCoreAssembly>true</IsCoreAssembly>
     <ProjectGuid>{ADBCF120-3454-4A3C-9D1D-AC4293E795D6}</ProjectGuid>
+    
+    <!-- Default interfaces are not officially supported in netcoreapp yet -->
+    <DefineConstants Condition="'$(IsPrerelease)' == 'true' and '$(TargetGroup)' == 'netcoreapp'">$(DefineConstants);FEATURE_DEFAULT_INTERFACES</DefineConstants> 
+    
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'netcoreapp-Debug|AnyCPU'" />
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'netcoreapp-Release|AnyCPU'" />