Framework Generation constants for Search-by-TFM (#9270)

* Added framework generation constant strings

* edited netcoreapp, xml docs

Author: advay26

src/NuGetGallery.Core/Services/AssetFrameworkHelper.cs

@@ -112,5 +112,19 @@ private static bool HasBuildItemsForPackageId(IEnumerable<ContentItem> items, st
 
             return false;
         }
+
+        /// <summary>
+        /// Framework Generation shortname identifiers used by the Search Service for framework filtering.
+        /// </summary>
+        public static class FrameworkGenerationIdentifiers
+        {
+            public const string Net = "net";
+
+            public const string NetFramework = "netframework";
+
+            public const string NetCoreApp = "netcoreapp";
+
+            public const string NetStandard = "netstandard";
+        }
     }
 }