[dotnet] Enable generation of runtime configuration file (#17840)

Set the GenerateRuntimeConfigurationFiles (GRCF) property to true 
to avoid warnings at build time + add test for change.

Diving deeper into the fix...
- This warning only occurs with .NET apps which is why GRCF
is only updated in the dotnet directory and not msbuild (legacy)
- After examining the binlog (see issue), it was found that the GRCF 
was contingent upon the HasRuntimeOutput property, which is only 
defined for executable projects. And in this case, the user's project
output type is library thus both the RuntimeOutput and consequently 
GRCF properties were not enabled.
- By setting the GRCF to true we can address the original warning of 
concern while ensuring the rest of the projects's behavior is not
altered
in mysterious ways (i.e. by touching the RuntimeOutput property or the 
project output type instead, these changes could have extraneous
effects).

Fixes #17543

---------

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

Author: haritha-mohan

dotnet/targets/Xamarin.Shared.Sdk.props

@@ -50,6 +50,8 @@
 			* https://github.com/dotnet/sdk/blob/c5a58bc6c3eb2b236b314e6d17a89a537459890c/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets#L102
 		-->
 		<_IsTrimmingEnabled>true</_IsTrimmingEnabled>
+		<!-- The GRCF property is contingent upon the HasRuntimeOutput property, which is only defined for executable projects. So, explicitly define GRCF for extension projects to avoid warnings at build time. -->
+		<GenerateRuntimeConfigurationFiles Condition="'$(IsAppExtension)' == 'true'">true</GenerateRuntimeConfigurationFiles>
 	</PropertyGroup>
 
 	<!-- Set the default RuntimeIdentifier if not already specified. -->

tests/dotnet/UnitTests/ProjectTest.cs

@@ -917,6 +917,10 @@ public void BuildProjectsWithExtensions (ApplePlatform platform)
 
 			var extensionPath = Path.Combine (Path.GetDirectoryName (consumingProjectDir)!, "bin", "Debug", platform.ToFramework (), GetDefaultRuntimeIdentifier (platform), "MySimpleApp.app", GetPlugInsRelativePath (platform), "ExtensionProject.appex");
 			Assert.That (Directory.Exists (extensionPath), $"App extension directory does not exist: {extensionPath}");
+
+			var pathToSearch = Path.Combine (Path.GetDirectoryName (consumingProjectDir)!, "bin", "Debug");
+			string [] configFiles = Directory.GetFiles (pathToSearch, "*.runtimeconfig.*", SearchOption.AllDirectories);
+			Assert.AreNotEqual (0, configFiles.Length, "runtimeconfig.json file does not exist");
 		}
 
 		[TestCase (ApplePlatform.iOS, "iossimulator-x64;iossimulator-arm64")]