[backend/federation] Bypass a bug in GTS that causes the type property to be invalid

Ref: https://github.com/superseriousbusiness/gotosocial/issues/1947

Iceshrimp.Backend/Core/Federation/ActivityStreams/Contexts/schema.json

@@ -0,0 +1,11 @@
+{
+  "@context": {
+    "schema": "http://schema.org/",
+    "PropertyValue": {
+      "@id": "schema:PropertyValue"
+    },
+    "value": {
+      "@id": "schema:value"
+    }
+  }
+}

Iceshrimp.Backend/Core/Federation/ActivityStreams/LDHelpers.cs

@@ -34,6 +34,13 @@ public static class LdHelpers
 				                                                      "Contexts", "toot.json")))
 			}
 		},
+		{
+			"http://schema.org/", new RemoteDocument
+			{
+				Document = JToken.Parse(File.ReadAllText(Path.Combine("Core", "Federation", "ActivityStreams",
+				                                                      "Contexts", "schema.json")))
+			}
+		},
 		{
 			"litepub-0.1", new RemoteDocument
 			{